Well, er... I haven't really found any other English-Language links about Tokyo-Underground yet, but I'll list some Japanese links soon.

  This is the official page for Akinobu Uraku, the manga artist!

STUDIO PIERROT'S TOKYO UNDERGROUND: Studio Pierrot has opened an official page for the anime series, in Japanese. There are character designs, cast and staff info, and episode summaries.

TV TOKYO'S TOKYO UNDERGROUND PAGE: This is yet another official page for the anime series, also only in Japanese.