This is the best website available for Pratchett fans, in the humble opinion of this reviewer. It has all the things you'd expect, all the official information, details on forthcoming books and so forth.
However, the real meat of the site lies in the extra information. The Annotated Pratchett Files which detail each and every reference and injoke in his books, the Pratchett Quote File featuring some of the best quotes from each book and the best of alt.fan.pratchett which details some of the funnier threads from that forum, some of which Pterry has contributed to himself.
This site also contains a library of Discworld filks, links to other Discworld sites and up to date information on signing tours and other Pratchett related social events.
Simply put, if you're a Pratchett fan, you should have this on your favourite lists. If not, reading this site may convert you.