This is a fast, multiplayer tetris of pure awesome.
Please note that to host an internet game you have to open port 12555
Click here to download Cultris
Click on the Image to download the game
Requires a very new version of Java (this is a link to Sun's website. Link may break with time, will always have newest version) as well as your latest video card drivers.
There's also a linux version which you can download here. I didn't have good luck in getting the linux version working, but you may have better luck.