Multris is a multiplayer internet Shareware Tetris Clone written by Olivier Chamley.
The latest,and final(?) version of it is version 1.0.
It supports up to 20 players, customizable backgrounds, handicaps and the such.
It is just a straight forward tetris clone in game play. To register the game, it costs about
twenty dollars u.s.

Here is a screenshot of it in action.