Tetris is a online multiplayer Tetris game. It was made as part of Ludum Dare 41 (this is one of many Ludum Dares by Tig / Meli). This is the 2nd game made by Atelier 801 that uses html5 canvas instead of flash, the first being God's Paw. May be part of Module 801 as the game uses a file "module801.js".
It was originally announced in a Transformice in-game server message, and then later added to the Transformice login page at the bottom right corner for a time.