Craig Shaw Gardner's Ebenezum/ Wuntvor trilogies are fantasy comedy.
Philip Jose Farmer (as Kilgore Trout) wrote Venus on the Half Shell as a sci fi comedy.
(On the topic of Kilgore Trout - that pseudonym is actually a character from several of Kurt Vonnegut's books - also mentioned in this thread.)
Yahtzee Croshaw (the Zero Punction guy) has also written several fantasy and sci fi comedy books - Jacques McKeown is one series.
Glen Cook, more famous for the Black Company series - also writes a fantasy comedy series - Garrett P.I..
Steven Erikson, more famous for the Malazan books - also writes a sci-fi comedy series - Wilful Child.
Also the Stainless Steel Rat series - it's not as slapsticky as Bill, but still comedic.