Lots of Valve stuff - Portal 1 / 2, the Half Life games, and Left 4 Dead 2 all go on sale for like a dollar very often. Team Fortress 2 is flat out free to play, if you're willing to either pay like $2.50 for an item to unlock your account or put up with a bit of f2p bullshittery. Put almost 4000 hours into TF2, it's my all time favorite game.