Diablo 3 Guide - http://www.diabloiiiblog.com
Current beta users reveals the experiences of Diablo III and how you can use the RMAH to your advantage.
GameMecca - http://www.gamemecca.net/
A community of gamers across all platforms.
VGchat - http://www.vgchat.com/
A community of video game fans. The forums cover a wide variety of topics, including music, movies, artwork and much more.