My son is only 2, but he really enjoys watching certain games and I'm working on getting him to play them a bit.
Gamecube: Legend of Zelda - He enjoys watching it.
Animal Crossing - Use this to teach him about different animals and the sounds they make, he calls out the names of the fish I catch, and we do some counting and letter recognition with it.
Mario Party: Not really a 2 year old kind of game, but it's a great multiplayer game for older kids and yes, even adults. Good clean fun.
Xbox: Midway's Greatest Hits - Teaching him to play Joust.

He mostly just likes pushing the button to make the ostrich fly.