Using fictious characters in rituals can be quite usefull for some.
Especially if you are pretty familiar with the deity/demon or whatever. Among fictious gods can also be counted the mythos of Great Old Ones and Elder Gods Lovecraft created.

However not all are suited for magickal use. It depends very much on the popularity of the system used.

Advanced Dungeons and Dragons

In RPG's such as AD&D the powers of the gods are clearly defined in the rules and since real players use them while they are playing and "believe" in them, they gain power - even if the players don't actually believe in them after the game. In this they are similar to us with paradigm shifting. As for fantasy books it can be said that many people get so into a book that while they are reading it, they believe in the characters and gods. Thereby they gain power. Especially when kids are reading them. If they like the books, they may start pretending to be the character they loved most. The better known the books are, the more likely is it that the enities become powerful.

In the AD&D system gods play an important role. Each character has a patron god. Clerics and Paladins usually call upon them for spells. So if you want to use them, you'd best assume the role of either a Cleric or Paladine. Use the holy symbol to call upon the god and summon it. In the Forgotten Realms there are also so called Chosen of the gods. I guess however that it would be better to contact with the chosen god before you declare yourself as his chosen <G> If you really need to you can bugger the Gods of Faerun into coming by commanding them via their superior, Ao (who also has a superior that wasn't yet named afaik). Generally however this is considered bad behaviour. To use them you'd best get a rule book where the gods are outlined, create a symbol of them and sanctify it. Use it to evoke/invoke them.

Dragonlance

Dragonlance gods from what I know of them wouldn't be too suited for anything since it's mainly the old good (Paladine), bad (Takisis), balance (Gilean) system. But then, those gods left had to leave at the end of the 4th Age.

Forgotten Realms

The gods of the Forgotten Realms universe can be much better used. A) there are more and b) they are better outlined in the fantasy books.

GREATER POWERS

• Akadi Elemental Air, Air Elementalists, Movement, Speed, Flying creatures
• Chauntea Agriculture, Plants cultivated by Humans, Farmers, Gardeners, Summer
• Cyric Murder, Strife, Lies, Intrigue, Deception, Illusion
• Grumbar Elemental Earth, Earth Elementalists, Solidity, Changelessness, Oaths
• Istishia Elemental Water, Water Elementalists, Purification through Cleansing, Wetness
• Kelemvor Death, The Dead
• Kossuth Elemental Fire, Fire Elementalists, Purification through Fire
• Lathander Spring, Dawn, Birth, Renewal, Creativity, Youth, Vitality, Athletics, Self-Perfection
• Mystra Magic, Spells, The Weave
• Oghma Knowledge, Invention, Inspiration, Bards
• Shar Dark, Night, Loss, Forgetfulness, Unrevealed Secrets, Caverns, Dungeons, The Underdark
• Silvanus Wild Nature, Druids
• Sune Beauty, Love, Passion
• Talos Storms, Destruction, Rebellion, Conflagrations, Earth-Shakings, Vortices
• Tempus War, Battle, Warriors
• Tyr Justice
  

INTERMEDIATE POWERS

• Beshaba Random mischief, Misfortune, Bad Luck, Accidents
• Gond Artifice, Craft, Construction, Smithwork
• Helm Guardians, Protectors, Protection
• Ilmater Endurance, Suffering, Martyrdom, Perseverance
• Mielikki Forest, Forest Creatures, Rangers, Dryads, Autumn
• Selune Moon, Stars, Navigation, Navigators, Wanderers, Seekers, Good Neutral Lycanthropes
• Tymora Good Fortune, Skill, Victory, Adventurers, Adventuring
• Umberlee Oceans, Currents, Waves, Sea Winds

LESSER POWERS

• Auril Cold, Winter
• Azuth Wizards, Mages, Spellcasters in general.
• Deneir Glyphs, Images, Literature, Literacy, Scribes, Pictorial and Literary Art, Cartography
• Eldath Quiet Places, Springs, Pools, Stillness, Peace, Waterfalls, Druid Groves
• Iyachtu Xvim Tyranny, Hatred
• Lliira Joy, Happiness, Dance, Festivals, Carefree Celebration, Contentment, Release, Hospitality, Feedom/Liberty; From Waukeen - Trade, Money, Wealth
• Loviatar Pain, Hurt, Agony, Torment, Suffering, Torture
• Malar Hunters, Marauding Beasts and Monsters, Blood, Bloodlust, Evil Lycanthropes, Stalking
• Mask Thieves, Thievery, Shadows
• Milil Poetry, Song, Eloquence
• Shaundakul Travel, Exploration, Long Range traders, Miners, Caravans, Wind Ghosts
• Talona Disease, Poison
• Torm Duty, Loyalty, Obedience, Paladins

DEAD POWERS

• Amaunator Bureacracy, Contracts, Law, Order, The Sun, Rulership
• Bane Strife, Hatred, Tyranny
• Bhaal Death, Especially violent or ritual death
• Ibrundul Caverns, Dungeons, The Underdark, Skulks
• Leira Deception, Illusion
• Moander Rotting, Death, Decay, Corruption
• Myrkul The Dead, Wasting, Decay, Corruption, Parasites, Old Age, Exhaustion, Dusk, Autumn
• Tchazzar Chessenta aka Tiamat
• Waukeen Trade, Money, Wealth (Missing)
  

ELVEN POWERS

• Corellon Larethian Arts, Crafts, Music, War
• Aerdrie Faenya Air, Weather, Avians
• Deep Sashelas Creation, Knowledge, Beauty
• Trishina Love, Fidelity, The Young, Play
• Solonor Thelandira Archery, Hunting
• Erevan Ilesere Mischief, Change, Rogues
• Fenmarel Mestarine Feral Elves, Scapegoats
• Hanali Celanil Romantic Love, Beauty
• Labelas Enoreth Time, Longevity
• Sehanine Moonbow Mysticism, Dreams, Far Journeys, Death, Transcendence
• Rillifane Rallathil Woodlands, Nature, The Elven Forests, Wood Elves
• Lolth Spiders, Evil, Darkness, Rotting Death, Decay, Corruption
• Eilistraee Song, Dance, Swordwork, Hunting
• Kiaransalee Undead, Vengeance
• Ghaunadaur Oozes, Slimes, Jellies, Outcasts, Rebels, All things Subterranean
• Vhaerun Theivery, Territory

Realms of Arkania

Another popular game (at least in Germany and Austria) would be Das Schwarze Auge (Realms of Arkania). Here's also a whole pantheon to choose from, with each god outlined to similar extend as in Forgotten Realms. Since I'd question the popularity and availability of the game I'd suggest not to use them if you want the rite to be successful - at least if its something that involves lots of power.
The gods of Realms of Arkania can be called like any others, there are no apparent guidelines for them. Among them would be:

• Praios: sun, justice, valor, symbol: Griphon
• Rondra: war, storms, symbol: Lioness
• Efferd: water, sea, rain, symbol: Dolphin
• Travia: guardian of home, patron of marriages, similar to the greek Hera, symbol: Goose
• Boron: death, sleep, dreams, forgetfullness, symbol: Raven
• Hesinde: wisdom and magic, symbol: Serpent
• Firun: winter and eternal ice, hunt, symbol: Icebear
• Tsa: life, healing, symbol: Lizard
• Phex: thieves and merchants god, symbol: Fox
• Peraine: fertility and healing, symbol: Stork
• Ingerimm: smithing, fire, creation, symbol: Hammer and Amboss
• Rahja: wine, sexual love, orgies, symbol: Horse
• Swafnir: guardian and thunderstorms, similar to Thor, symbol: Whale

Then there's one without a name, a god of darkness. His symbol is a sleeping man
All of those gods here have a symbol, similar to the AD&D gods by which they are called.

Midkemia

Other useful gods can be found in various Fantasy books like the Midkemia books by Feist. He even has a similar believe system as in the chaos approach. The gods there are fed by believe and may die when the believe stops. Since Midkemia started out as an alternative to the complicated D&D it is extremely detailed. All the gods have certain powers and uses. Lims-Kragma for example would be a Death Goddess. Ishap for healing.

The gods of Midkemia and Kelewan are basically manifestations of primal powers that fed of believe of the various races. So a god of Midkemia would be the same as one of Kelewan, the only difference being would be different names and looks. The belief shapes them here. In his latest series (Serpent War) this is outlined quite good (still waiting for that damn last part). Gods here can die, but the primal power remains, so the gods can be resurrected but this takes time and many believers.
The gods in this fantasy world are called similar to AD&D which is no wonder since it first was a game system. So here too you could assume the character of a cleric/priest to the god. To call Ishap however would take a recreation of the Tear of Ishap which holds his power (or that of all the other gods that spare energy so since the god perished in the Chaos Wars against the Valheru.

The Two Blind Gods of the Beginning were replaced by Ishap's "kin." They were basically just forces of the Universe. Then the Chaos Wars came on and these forces established a domain. The Valheru fought against these forces and lost. Then humans and dwarves and the other races came and shared the planet with the elves and dragons. These peoples began to attribute physical traits to these forces, thus Lims-Kragma is perceived kind of like Lady Death in the Kingdom. It's not what Lims-Kragma wants to look like because Lims-Kragma doesn't really care about appearance, but that is the way most Kingdomers want her to look like when they see her, so she does.

The gods' power comes from themselves. They are defined by the people. In Midkemia, there's a force that evolved that is perceived by humans as Lims-Kragma, the Goddess of Death. If humans, for whatever reason, decided that death isn't the province of Lims-Kragma, but instead part of Killian's duties, then Lims-Kragma would phase out. There would still be a force of death, but there would be no "Lims-Kragma personification" of that force; it would be Killian, the Some-Times Goddess of Death.

Now in a case like Sarig, the humans just decided that NO god was a force of magic. So the personification of Sarig, God of Magic, instead of become the province of another god, phased out completely. But the FORCE of magic didn't disappear, it was just "forgotten." Macros worked to bring the Greater Path to Midkemia so that there would be an increase on magic used. Then, after there was a good-sized population of magic-users, he probably would have said, "Okay, now all your magic comes down from Sarig, the God of Magic. Every time you cast a spell, it's like worshipping him." So the humans cast spells and each time they do it, they think, "Oh yeah. This is also god worsipping. Cool." They picture a god helping them out or something, and sooner or later (okay, just later) the "rumor" spreads and poof! Sarig is the full-fledged God of Magic again.

Okay, just to wrap up. There are forces out there. They have a great deal of power, but they aren't "they." They just are. Think of them as big masses of nothing. People associate things that happen with these forces. At first, they begin to imagine that SOMEONE makes these things happen. Then they start to give physical attributes to the unseen forces. Then a group of scared people get together and form a temple to please these gods so nothing bad happens. Finally, the entire population has a perception that there is a certain god that makes certain things happen, and this god looks like this and acts like that, and you must do this to make him happy and don't do that unless you want him mad.

Controller = Greater Gods:

• Abram-sev: The giver of laws; the thunderer
• Ev-dem: The worker from within; the calculator of odds
• Graf: The weaver of wishes
• Helbinor: The abstainer
• Ishap

Lesser Gods:

• Silban [ Ma-wan-ta ]: the earth mother; the bringer of harvests; sleeper without dreams
• Sung [La-Timsa ]: the white; the follower of the one path
• Ka-hooli [ Albar ]: the howler after fugitives; the tireless pursuer; the warrior god of vengeance; the unraveller of mysteries into truth
• Astalon [ Ynothanos ]: the just; the builder

(Generally neutral)

• Ban-ath [ Dav-lu ]: the artful dodger; the prankster; the night walker; the silent thief of precious commodities
• Kilian [ Ka-Ya-taha ]: the singer of green silences; the gatherer of quiet pleasures; the goddess of love
• Tith [ Onan-ka ]: the war god; the tactician; the happy warrior
• Dala [ Indarna ]: shield of the weak; goddess of passive strength; the protector

(Chaotic gods - as in order)

• Ruthia [ Ilyandros ]: the dancer through men's hearts; the goddess of loved scorned; the lady of luck
• Guis-wa [ Fimbulstarn ]: The bayer after the moons; the red-jawed hunter; the reveler in forbidden knowledge; the wanter of all things
• Prandur [ Jehan-suz ]: the burner of cities; the spreader of confusion; the light bringer
• Lims-Kragma [ Zandros ]: The drawer of nets; the mistress of death

Chaos

The gods from the Chaos series by Louise Cooper (3 trilogies - Time Master, Chaos Gate, Star Ascendant) can also be used, which would be my favourite. It isn't as detailed as the others mentioned above, but I still have a soft spot for Tarod <g>. In her system there are 7 gods of Chaos and 7 of Order. one god for each element: fire, water, earth, air, space, time, life and death (life and death is combined in one god - the 2 rulers).

The Chaos gods of these books usually reside in their realm and don't really bother with mortals (as do the order gods). To contact Yandros and his brethren the gods left a Chaos Gate for the mortals that is activated by the Speaking of the Way (basically a Vortex opening - a vortex here is a storm that comes directly from the realm of Chaos). A version of the Speaking of the Way can be found here. The Order gods or more precisely Aeoris is contacted by a High Triumphirat (High Initiate, Leader of the Sisters and the High Margrave) by opening a certain white box that summons him. Working with Aeoris and his brethren would be rather like working with any Judeo Christian god aspect (xtian, jewish and musli), he's commanding, his way is the only right etc. So there's no real use for him. The chaos gods however are way better to work with. Last year I performed a couple of workings with Tarod and Yandros and I do like them.

• Yandros: Life and Death
• Tarod: Time
• Cradon/Cyllan: Space
• Iometh: Earth
• Sarisan: Water
• Lermuar: Fire
• Gilmask: Air
  

Belgariad and Malloreon

Then of course would be the gods of Eddings creation. Aldur would be a god of research and wisdom, Belar of strengh and war, Mara of lust (and sorrow) etc.

Eddings gives no real summoning of any god in his books. The gods here are like humans, with prejudices and all. It might be funny tho to work with Belar (likes fighting and a good feast) or Mara (liked orgies) for example. Eddings characters usually just talked with the gods in a chatty way. Torak however might want some sacrifices (get the knives ready <g>).

Choosing characters instead of demons or gods to use in rites might not be overly wise.
Usually those characters are at the start pretty weak and gain only limited power. Further there's the trouble with heroes sacrificing everything for the greater good.
And they are based on humans (or any other species that like elves, dwarves etc) which is also rather limiting their uses. Also they have no specified powers. If you choose a mage for example he can cast a lot of spells depending on level, but what real use would that be in a rite, working or spell? And usually there would always be an entity from the system you chose it from that is more powerful. There would be the possibility to create in your favourite system a character and push it up as high as possible. But since you (and maybe those who are playing with you) would know about that character it's powers would be limited and it would only become a servitor.

So if you really want to use fictious creatures, go for gods and demons, but not for human/elven/dwarven/halfling characters. And you'd better pick it from a popular system.