I've played Tetrinet off and on since 1996! It's so simple and yet geeze, I bet I've devoted a few thousand hours to it (and that's a lot when you think about it).
But for something truly silly (that doesn't seem to have been mentioned) <a href="http://jet.ro/dismount/">Porrasturvat!</a>. You basically push a 3D guy down a lot of stairs and get points for how much damage you do to his body. It's even better than it sounds. My friends and I played this a lot one Winter... it was rather odd.