There are basicaly four threads
Rincewind, Vimes, Death/Susan and witches. Of course, they intertwine and cross-pollinate a lot.
The Rincewind is the original thread, and it also happens to be the one I like least. "The Color of Magic" and "The Light Fantastic" are the first to books there, if I remember correctly. It goes on to Equal Rites, Sourcery, Eric, Interesting Times (that one is good), The Last Continent and I guess The Last Hero.
Vimes starts with Guards,Guards and goes on to Men at Arms, Feet of Clay, Fifth Elephant, Jingo, Night Watch, Thud.
Withes (I like that one qute a lot) starts with Wyrd Sisters, then Witches Abroad, Carper Jugulum, Lords and Ladies, Maskerade.
The Death/Susan series starts with Mort (not sure if you really want to start there, but that's chronological order), then goes to Reaper Man, Soul Music, Hogfather, Thief of Time.
And then there are somewhat standalone items: Small Gods, Pyramids, The Truth (better read somewhere in the middle of Vimes series), Going Postal (extremely recommended to geeks - it's got GNU in it! Better read after Feet of clay), Monstrous Regiment (better read after Fifth Elephant and The Truth).
