CLAMP-net - CLAMP's official page. Japanese language only.
NHK - The official Tsubasa anime site. Japanese language only.
Shonen Magazine - The homepage of the magazine that publishes Tsubasa. Japanese Language.
Holic-Tsubasa - The official site for the xxxHolic/Tsubasa movie. Japanese language only but click on the buttons to see a trailer ^^

fanfiction.net's Tsubasa page - bunches of Tsubasa fics. I make no guarantee of their quality >>
fanfiction.net's CLAMP page - Lot's of Tsubasa fic on here, too.
Noire Sensus - A great yaoi fanfic archive with some Tsubasa fics.

Blue Skies - Awesome FayLove site ^^
Wing Journey - A Tsubasa fan site
Hitsuzen - Another Tsubasa fan site.
KuroganeUke - Loads of fanart, some smut. Japanese language.

Hyu - The place to find all the translations of the Tsubasa chapitres by a lady who (get this!) actually knows how to write! Even if you read the Del Rey versions, please take a look at this.
Eomaat - A wonderful site that uploads scans of the new chapitres as the webmistress gets her copy of Shonen Magazine.

Chiaroscuro - The offical KuroFai fanlisting
Tattooed!!! - A Fai fanlisting

TsubasaRC - A Tsubasa community on LiveJournal.
KuroxFai - A KuroganexFay community on LiveJournal.
Tsubasa Media - A great place to get Torrents for the subbed anime episodes.

Black Steel and Faith - A KuroxFay Yahoo group run by mahself ^^

Fai Lovers - A Fay fanclub on DeviantArt
Kurogane Fans - A DeviantArt group dedicated to Kuro-chii
KuroxFai - A Kurogane/Fay DeviantArt group
Tsubasa Yaoi - A DeviantArt group dedicated to Tsubasa yaoi pairings of all kinds.
Kurogane Club - Another Kurogane club on DeviantArt ^^

eBay - The online auction site. I'm putting a link on here 'cause that's where I get most of my Tsubasa stuff.
YesAsia - An online store that sells all sorts of Asian goodies, including all sorts of lovely CLAMP stuffs.
J-List - I don't think there's any Tsubasa stuff but I love J-List, thus it gets linkage :)