This is not a comprehensive list, by any means, but in my humble opinion, these are the sites that best represent the Tsubasa fandom.

Communities and Mailing Lists - English
CapturedWings.net Forums
CLAMP_Now

Fansites - English
Black Steel and Faith - KuroganexFai Fansite
Blue Skies - Fai Shrine
Clow Legacy
Intensity - Kurogane Shrine
Loop - A Tsubasa Chronicle Anime Companion

Raw Scans & Translations
I Hyu Because...Yes
Room of Requirement
TSUBASA SCRiPTS RESERVoir