Tetrinet is a great multiplayer game for up to 6 players.
You can play either in team or alone. There are several
ways to fasten your opponents 'death' and to make yours
last longer. You can use Earthquakes, Switch fields, nukes,
Random blocks clear, and so on. Learn to use them right
and you will accomplish better.

TetriNet is written Craig Moksnes, on IRC StOrmCat.
It is distributed as freeware, in another word, its FREE.