The Chaos Sanctuary - Official D2 site. The biggest one I know.
Blizzard Entertaiment - D2 Developes site.
If u want to buy diablo... - Blizzard online store.
A site about Diablo, Hellfire and D2. - Some cheats
Russian D2 site. If u have cyrillic fonts - go ahead.
D2 Fan site - the biggest fan site I have ever seen! Its bigger than b.net
D2 Bugs - no comments
Yegg's tavern Anyone knows who is this Yegg? :) Such a funny name.

Thats almost all i know. If you know some more sites about D2. Send a URL to me. I`ll post it here.