I just an hour ago finished reading Good Omens, by Terry Pratchett and Neil Gaiman, and I have to say it might be the funniest I've ever read. The footnotes on their own are golden, and then about halfway through... actually almost exactly halfway through, I literally laughed out loud because "it" was so unexpected.
I've also just started Pratchett's The Color of Magic and it's on a similar path.
Other than that, I'll say Jim Butcher, with his Dresden Files, on occasion have made me chuckle.