Well, I’m not quite sure if the OP is looking for comic fantasy or just light-hearted fantasy with some comic elements, but I’ll add John Morressey’s Kedrigern novels. They’re generally OOP, so hard to find, but Meisha Merlin has started reprinting omnibus editions. Something happened, though, and they’re nearly a year behind on printing the third volume.
Tanya Huff’s Summon the Keeper was funny fantasy, very enjoyable.
I keep forgetting to find the next in that series.
And for the younger set, though I enjoy them as well, Jonathan Stroud’s Bartimaeus Trilogy is a lot of fun.
Tom Holt: Expecting Someone Taller (Ring Cycle), Flying Dutch (Flying Dutchman) and a few others. Really made me laugh out loud a few times. And I don’t do that.
Glen, not Rick Cook. Right. Rick Cook does the Wiz Biz novels, which are funny if you’re a computer geek who likes fantasy.
SF-wards, the Plutonium Blonde by a man I misrember, and the sequel… about quadruplets?
In addition to Terry Pratchett and the Myth series from Robert Asprin, Roger Zelazny wrote a pair of quite funny fantasy books: Bring Me The Head Of Prince Charming and If At Faust You Don’t Succeed. They’re reminiscent of Good Omens, though not as good.
A third Zelzany, illustrated by Gahan Wilson: A Night In The Lonesome October.
So very good.
The Chicks In Chainmail books are a series of short story anthologies. They’re all humorous or at least semi-comic tales of fantasy warrior babes, some in metal bikinis and some in more sensible armor. (Actually, at least one story deals with the difficulties Amazon types face in trying to find blacksmiths that will make women’s armor that covers all the vitals!) It’s been years since I read the books, but picture something in the spirit of the lighter episodes of Xena: Warrior Princess and you’ll probably have the general idea.
A third book? ::ears prick up:: To the used bookstore, Robin!
Though Diana Wynne Jones is predominantly an author for young adults, Deep Secret is an excellent read. Though I would hesitate to call it “comic”, the author’s wit and humor is definitely several cuts above the usual fantasy fare.
Both of these books, along with A Farce to Be Reckoned With, were co-written with Robert Sheckley, a long-time master of the comic f&sf story. Zelazny had a fine sense of humor, but you have to give Sheckley a lot of the credit for the comic tone.
And if you’re going to mention Gahan Wilson, remember that he wrote as well as illustrated. You’re not going to find anything much wackier than Everybody Favorite Duck.
They sound interesting - I’ll have to check them out. Thanks for the recommendation.
Fair warning – first one was decent, but A Long Hot Summoning didn’t really do it for me. Overall, I liked her Quarters books more.
I get to be the first one to bring up Mary Gentle’s Grunts? Woo hoo! Uh, this book is about an orc horde that discovers modern military weapons and the knowhow to use 'em. It makes fun of dozens of high fantasy cliches from the orc’s point of view. 
Voyage of the Shadowmoon and Glass Dragons by Sean McMullen.