I can't say I find Pratchett to be extremely funny, but his stuff is a decent read generally. I have quite a few paper versions, and I'm trying desperately to find the black cover editions locally.
Asprin's Myth stuff has so far been enjoyable for me (just first two books), but like some Pratchett, it's more smiling-amusing rather than hilarious. The Wodehouse stuff I've read so far (Blandings Castle stuff mostly) has been similarly smile-inducing.
Jasper Fforde's Thursday Next series (at least the first two books) have gotten some chuckles out of me, though they're a different brand of comedy.
I did laugh out loud once or twice at Christopher Moore's "Lamb". I have other books of his that I'd like to dive into, but I'm always worried that I'll end up disappointed after a strong showing from Lamb.
This question on Amazon offers some interesting results that I haven't seen mentioned before, and would like to check out myself.