Fizban is a manifestation of the god Paladine in the Dragonlance universe, originally created by Margaret Weis and Tracy Hickman. He first appears in Dragonlance Chronicles where he, for whatever reason, chooses to help the Heroes of the Lance by pretending to be an absent-minded wizard with a silly name. During that series, Fizban befriends Tasslehoff, a very child-like kender, and thereafter decides that the Fizban persona is his favourite.
Suspiciously similar characters named Zanfib and Zifnab appear in the authors' other series The Mantle of Kendis-Dai and The Death Gate Cycle respectively. Both characters' names are anagrams of "Fizban", are described as looking similar to him, and have the same personalities. However, Hickman has pointed out that these characters are owned by himself and Weis, while the Dragonlance character Fizban is owned by their employer TSR (and nowadays, owned by Wizards of the Coast).
Interestingly, Zifnab says in the the last book of the Death Gate series that he was in the world during the sundering -- an event which turned a stock fantasy world very similar to Dragonlance into the several networked worlds of Death Gate Cycle. This Dragonlance-esque world is itself implied to be a future post-nuclear war version of our Earth, and Zifnab's repeated references to real-world pop culture (like Godzilla) imply that he may have been present in our present-day world.
Fizban also makes references like this, which links the characters even more together. If you accept that they're one and the same, and remember that Fizban is a human form of Paladine, this means that one of the key Dungeons and Dragons deities existed as a god today.
What's even more interesting is that The Death Gate Cycle doesn't explicitly have any gods at all -- in fact, one of the recurring questions in the series is whether God exists, and if he doesn't, whether the magic-using humans should be considered gods. In the epilogue, Zifnab decides that he must be God himself, since he can remember so many different worlds, but he seems confused about it.
Whether he's feigning that confusion or he's genuinely forgotten, who knows? Maybe Death Gate's world is set so far in the future that the original gods have all died, thus explaining the themes of the books -- the old world is dying off, and humans have advanced themselves (indirectly through technology, even) to the status of gods.
Or maaaaybe I just read too many fantasy books as a kid and thought about this way too much.