Case's TetriNET ladder (IRC) [dead]
The French TetriNET ladder [hard to believe, but people play here]
QuadraNet [hm, I wonder exactly what this one is used for]

Clans:

Team LionZzZ
Team Flatline
Team SilverDragons
Clan HoRRoR [no link]
Clan DarkInferno
Team Legacy
Clan DdP
Team RecogniZe [includes a working Quadra ladder]

HELLFIRE clans:

Starcraft clan HELLFIRE and #2
Quake clan HELLFIRE #1 and #2
The old HELLFIRE homepage

Tetris webpages:

tetris.com
The official Quadra home page
tetrinet2.com
roncli.com
The unofficial Gameboy Tetris home page
netris.org
tetrinet.org
plibble.com
Planet TetriNET
Tetris research page [featuring a nice little JAVA applet]
all right, this applet is not so nice, one day I tried and managed to beat the highscore and the highscore submission didn't work! AND I beat it BOTH on IE and Netscape P: (life? what life?) 
TetriNET2 official message board
TetriNET server listing

Main.