A mini-bibliography:
ISBN:0060012358 The Amazing Maurice... ISBN:0061020397 Carpe Jugulum ISBN:0061020710 The Color of Magic ISBN:0061020699 Equal Rites ISBN:0380821214 Eric ISBN:0061057649 Feet of Clay ISBN:0061020648 Guards! Guards! ISBN:0061059056 Hogfather ISBN:0061020400 The Fifth Elephant ISBN:0061056901 Interesting Times ISBN:0061059064 Jingo ISBN:0061059072 The Last Continent ISBN:0060507772 The Last Hero ISBN:0061020702 The Light Fantastic ISBN:0061056928 Lords and Ladies ISBN:006105691X Maskerade ISBN:0061092193 Men at Arms ISBN:0061020680 Mort ISBN:0552144290 Mort: The Play ISBN:006102063X Moving Pictures ISBN:0061054895 Soul Music ISBN:0060013117 Night Watch ISBN:0061020656 Pyramids ISBN:0061020621 Reaper Man ISBN:0061092177 Small Gods ISBN:0061020672 Sourcery ISBN:0061031321 Thief of Time ISBN:0380818191 The Truth ISBN:0060012366 The Wee Free Men ISBN:0061020613 Witches Abroad ISBN:0061020664 Wyrd Sisters
Disc World inspired
ISBN:0091865158 The Science of Discworld ISBN:0385600054 Nanny Ogg's Cookbook
Non-Disc World
The Carpet People Truckers Diggers Wings Strata Dark Side of the Sun
The Johny books
Only You Can Save Mankind Johny and the Bomb Johny and the Dead
Short stories (please complete details)
"Troll Bridge", featuring Cohen the Barbarian and trolls, in a Middle-Earth inspired anthology something about death turning up at a retirement costume dance Hollywood Chickens (see http://www.ufbs.co.uk/dwm/dwm0011.html) an essay on chickens crossing a highway (very, very funny)
Others
Good Omens [ISBN 0441003257] or [ISBN 0425132153] (with NeilGaiman) Discworld roleplaying game (formerly GURPS Discworld, Steve Jackson Games) with various authors
www.lspace.org
General discussion
Terry writes brilliant humorous fantasy. His book The Colour of Magic is on my list of five best fantasy books of all time. -- Kent Beck
I second that emotion -- Alistair Cockburn
I agree. What are the other four? -- Martine Devos
See Funny Fantasy for one list.
I prefer "Mort", but agree that Pratchett is great. -- Betsy Hanes Perry
"Small Gods" is roaringly funny. -- Dave Smith
I haven't read Hogfather yet, but I can recommend all the other books. They are extremely funny. If you get a chance to read one (or more) do so!
Hogfather is quite good as well - Pterry has cost me a fair bit in shipping charges for books, since they seem to be so difficult to get here in the states. Gotta run off and get Jingo and the Last Continent...
I don't know if it's my cultural ignorance or what, but the books seem less funny as date of publication increases (this is not meant to be a negative comment - I still find them to be riveting reads and highly recommend them). I laughed out loud at the Colour of Magic and the Light Fantastic, giggled through the ones through Pyramids, and snickered at next few. I'm not sure what this says about anything...
-- Bill Jamison
This is surprising to me as I find his books the opposite. The Colour of Magic, and The Light Fantastic seemed rather flat when I first read them, and they still do when I go back to them. Personally I consider Interesting Times and Hogfather to be his best, closely followed by Mort. And of course being an Australian enjoy The Last Continent. -- Andrae Muys
Someone made the observation years ago on alt.fan.pratchett that people tend to consider the first Pratchett book that they've read as the best. That somewhat holds true in my case - I consider Mort to be one of Pterry's best.
-- Alex Valdez
Fantastic, it's great to see there are like minded individuals out there. I've got 20 of the 22 Discworld novels and they're all hilarious! One for the American audience has to be "Moving Pictures" and any Australians out there may find "The Last Continent" amusing. -- Stuart Barker
The language spoken by the Celts in "Soul Music" is Welsh by the way.
("Sioni bydd dda" = Sonny B Goode, "Imp y Celyn" = Bud The Holly,...)
-- some guy
Really, you should read both "Jingo" and "Night Watch", they're really mature writings and have really good cynics about politics.
-- Matteo
Good Omens
Also check out his collaboration with Neil Gaiman, "Good Omens", hands-down the funniest book about the Apocalypse. -- Betsy Hanes Perry
Actually, I'd include Good Omens as the hands-down funniest book about the creation, all of human history and the apocalypse all three! -- Kyle Brown
"Good Omens" was a great tale of a boy and his dog... SF Site is a great place to find reviews of science fiction and fantasy books. It's led me to several books that I never would have picked up otherwise. -- Katy Mulvey
The first Pratchett I read was Equal Rites, which I started reading in a supermarket that happened to have one of those rotatey book stands to the annoyance of my parents who were trying to buy food for us all at the time. I went off and bought CoM & tLF a few weeks later to read them first. While they're not the greatest, they're still good. I don't buy Mr Pratchett's books anymore - I'm waiting for him to stop writing diskworld novels, which frankly started feeling a little tired at about #12...
Good Omens is a nice standalone read.
-- Katie Lucas
The Discworld RPG is not by Pratchett alone, but it is 1) very funny 2) a good "behind the scenes" look at the Discworld 3) an excellent roleplaying game. -- Lorenzo Gatti
See also: www.lspace.org 
and www.sfsite.com
See original on c2.com