Official Discworld site.
I love this man's artwork. He has illustrated the Pratchett Portfolio, the AMCW 1999 Diary, and Last Hero, among other works, and here on his website many lovely pictures may be found.
If you know it, you know it. If you don't, you should. Go and look.

The City Watch
Want to roleplay as a member of the Watch? This is the place to go!

Bucked Up
Another Watch site. Seems to have the same goals as me, only actually achieves them. I'm going to have to update in order to hold my own! ;)

The Turtle Moves
News, competitions and a very shiny forum.