Well, it's hard but I think this site got some good info.
List That link is basically just a list of the Discworld books, not even complete I believe but the first books are pretty vital to start with considering all the different story lines in later books.
Here's another view on the Discworld, one which tells what book best to read first if you are interested in either Wizards or the witches or the Watchmen:
Order It tells you what book to read first if you wanted to stick to the Watchmen for instance, and in what order the rest should be read then.
If you liked Rincewind, definitely check out 'FAUST - Erik' and ofcourse my favourite 'Sourcery'.
edited to add: both links don't appear to work very well, strangely enough :(. Nevertheless, for the second link, go to the main page and just search for Pratchett, the fifth result is the page you need to see the reading order.
edited again: the first link is corrected and should work properly after using your very handy tip! Thanks for that, I'll try and remember it.
The second link is harder, I'll keep working on it as it is a rather handy site.
Edited, Sat May 14 20:00:46 2005 by Zieveraar Edited, Sun May 15 05:42:06 2005 by Zieveraar Edited, Sun May 15 05:44:11 2005 by Zieveraar