Well, I can help you guys out some. Although chronological order is not very important, things that have happened in one book often carry a bit over into other books. There are different "series" within Discworld of course.
The Watch series starred by Samuel Vimes is probably my favourite. Once you get into Men At Arms, it is lovely. There's also a real sense of progression over the course of these books. Sam is a drunkard who picks himself out of the gutter a bit in the first ones, then he gets married and they put the watch back together etc. The books are (chronological order) :
Guards! Guards!
Men at Arms
Feet of Clay
Jingo
The Fifth Elephant
Night Watch
Thud!
Warmly recommended
For newcomers, I'd also recommend the Moist Von Lipwig series which is just two books so far, "Going Postal" and "Making Money". Then you'd start with Going Postal!