Hogwarts Express MUSH

JKR.com J. K. Rowling's official website.
The Harry Potter movie site The official site of the Harry Potter movie, at warnerbrothers.com.
The Harry Potter Lexicon The self-proclaimed "ultimate Harry Potter reference."
The Leaky Cauldron An excellent site for Harry Potter news, pictures, and more.
Another Potter news, information, and discussion site with a weekly podcast.
Rane's MU* Resources A page of... well, MU* resources.