Dan Savage’s books The Kid and The Commitment are laugh-out-loud funny. Also very touching.
To me his early books are funnier. His later books are better. If that makes any sense to you.
It makes perfect sense. The early books were supposed to be funny. The later books are Literature, which can be humorous (and are), but have an underlying purpose and theme that has more gravitas than mere parody/satire.
Another good series that mixes humor with straightforward SF action are the Sten books by Chris Bunch and Alan Cole. These are worth seeking out, if only for the spotted snake joke, which takes 5 books to tell.
The scene with her dad proving a point about their beagle ( or whatever)not being a particularly noisy barky dog to a complaining neighbor and bringing in all his friends dogs and letting loose a raccoon or something, made me nearly pee myself.
Oh good. Those are two I’m picking up at the library tomorrow. 
Gil’s All Fright Diner by Lee Martinez is comedy and very light horror. More along the lines of I know what you did last summer and Another Scary movie.
It was a delightful read ( this coming from a woman who is too scared to watch horror movies or ready scary books.)
I read it long ago, but Richard Brautigan’s A Confederate General from Big Sur made me laugh out loud. Something about falling through the wall that wasn’t there…
Edward Abbey, Monkey Wrench Gang and, when it’s not making you cry, A Fool’s Progress .
Oh, someone asked upthread about reading ** Jasper Fforde** in order. It’s not necessary, but it is helpful.
I’ll second a few already mentioned: Wodehouse, Sedaris, Catch-22.
Also:
[ul]
[li]Nobody’s Perfect, a collection of Anthony Lane’s New Yorker movie reviews[/li][li]Paperweight, a collection of newspaper columns and radio broadcasts by Stephen Fry[/li][li]Almost anything by the late, great Molly Ivins[/li][li]Various novels by Vladimir Nabokov or John Barth (the wordplay-geek in me reads them in open-mouthed awe)[/li][li]MOO, a university-satire novel by Jane Smiley[/li][/ul]
Jasper Fforde has two “series.” You can start with either one, but within each, the books should be read in order. The “Nursery Crime” books (imagine Who Framed Roger Rabbit with fairy tale and nursery rhyme characters instead of toons) start with The Big Over Easy. The Thursday Next books (imagine, oh, classic English literature, add in crime fiction, chick lit, sci-fi, Lewis Carroll, Douglas Adams, Buffy the Vampire Slayer, and about half a dozen other things thrown in a blender—yeah, I know, it sounds like a mish-mash, but Fforde makes it work beautifully) start with The Eyre Affair.
I’m quite fond of Emma by Jane Austen of course. 
Jasper Fford’s Next novels are some of my favorites. worth owning, not just reading.
If you don’t mind reading YA lit, the Artemis Fowl books by Eion Colfer are quite fun. Read in order; start with Artemis Fowl. I got my oldest hooked on the series, so I can buy them for her and still have them around for me.
One of the other posters mentioned Mark Twain. Seconded loudly here. “Classic” (as that one might feel undue pressure that one really should enjoy it) but wonderful regardless. The one where he sets out to confuse some poor schmuck newspaper interviewer is priceless.
Dave Barry’s Book of Bad Songs by Dave Barry, as mentioned.
The Drake Majistral books by Walter Jon Williams; The Crown Jewels, House of Shards, Rock of Ages; humorous sci-fi.
The Ebeneezum and Wuntvor fantasy parody series by Craig Shaw Gardner ( "Why should we do what you want ? “Because I am the dragon. You . . . are NOT the dragon. Any questions ?” )
The Cineverse Cycle also by Craig Shaw Gardner.
Terry Pratchett, like half the board apparently. 
The Summoning series by Tanya Huff ( “Never tell Egyptian gods they remind you of cartoon characters.” )
The Samurai Cat books by Mark E. Rogers ( “How can you tell he’s a Hollywood Ninja ?” “Because it’s underground and he’s wearing all white. If it was aboveground in the snow, he’d be wearing all black.” )
Ummmmm, it’s good, and the first one ( Blue Moon Rising ) starts out lighthearted enough, but overall it’s rather dark. There’s a good bit of humor mixed in with the grimness, but there’s a lot more adventure, blood and tragedy than there is of the humor.
Just thought I’d warn anyone who decides to read it, since I read the first assuming from the cover and blurbs that it was a lighthearted fantasy, and it was at first - and then it got grimmer, and grimmer, and I spent the rest of the book wondering when it was going to lhghten up. It didn’t. Good, and very funny in places, but grim for the last 2/3rds. The rest of the series tends that way as well.
I second all of these.
I just started Good Omens as it came in from the library. Enjoying it and chuckling so far in the first chapter.
I really feel like the odd man out - everybody in my family (men and women, young and old) who has read her stuff loves it. My ex-g/f tried to get me into it and it did absolutely nothing for me. Same thing with “Cryptonomicon”, she found it hilarious, I felt it was the result of a drunken bet that the author could turn a bad 200 page story into ten thousand pages.
What makes me laugh? Dave Barry & Carl Hiassen. Helen Cresswell’s “Bagthorpe Saga” when I was a kid, also the “Freddy The Pig” stories.
Recently picked up a copy of HG Wells’ stories and the beginning of “Food Of The Gods” had me laughing out loud when I went out for breakfast.
I’ll admit that Pratchett doesn’t do it for me at all (although I suspect that around here this is like saying I shoot kittens for fun). He’s like Piers Anthony without the pedophilia. But no person’s literary education is complete without reading some Wodehouse. I think there is general consensus that his later stuff is not as good as his early- to middle stuff. Try something like The Mating Season. Frikkin’ genius.
It’s not for everyone, but Steve Martin’s Pure Drivel is a “never-in-public” book for me. I once read Writing Is Easy! aloud to a writer-friend of mine and we both cried with laughter. Even now, all I have to do is think the words “Red Guy”, and “Frappe”, and I giggle.
Also, Emma Who Saved My Life, by Wilton Barnhardt. Susan and cucumber dip, and 4th of July at the Jersey Shore, and herpes and Rigatonio… I feel like I was there. I think that book is responsible for my policy of not taking anyone seriously.
The James Lileks books The Gallery of Regrettable Food and Interior Desecrations, where he reprints and comments on 1940s and 1950s cookbooks and 1960s and 1970s Home Decor magazines, respectively, never fail to crack me up. I haven’t read his other books, but suspect they wouldn’t work for me, either, based on his website. Some of the stuff at www.lileks.com is hilarious (that’s where these books got their start), but others leave me unmoved. I leaned of them through this Board.
John Barth- The Sot Weed Factor
All Hunter Thompson books. His style was amusing without all the scrapes he kept narrowly avoiding.
I second that. It has me in stitches every time.
Also a big fan of Connie Willis’ “To Say Nothing of the Dog.”
Damn it. I came in here to list this one. Freeking hilarious! He’s one of my all time favorites!