Games-Workshop - The people behind it all!
Death Guard - A good site with lots of info on Nurgle's Traitor Marines
Deathkamp Drones - The Iron Warriors page of a friend of mines! These guys are sick!
Damage Inc. - A great site for Online RPG placed in the Warhammer 40000 universe
Index Astartes - An excellent board for all manner of Warhammer 40000 topics. Join the Family!