Using fictious characters in rituals can be quite usefull for some.
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 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.
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.
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.
Then there's one without a name, a god of darkness. His symbol is a sleeping man
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 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:
(Generally lawful - as in order, not attitude)
(Chaotic gods - as in order)
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.
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.
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.