For ones I would reccomend...
King Arthur's Gold Classic - I was hooked on this for ages before the Beta release came out (You have to pay for the Beta, but Classic is free) - best 2D mix of Terraria and Team Fortress 2 I've ever seen!
Super Crate Box - A classic. The game is relatively simple: Collect crates while killing enemies, but it's so well-designed and addictive that you'll be hooked for ages. Try to beat my score of 160!
Quake LIVE - Online, free-to-play Quake 3 Arena. Need I say more? It's very well-made, and although there's a few "free restrictions", it doesn't stop it from being a pretty fun game to play.
Spelunky - A fun platformer with a roguelike twist. The levels are randomly generated, and EVERYTHING wants to kill you. If you die, it's right back to the top level! There's multiple endings too, and it's quite a fun challenge to try to get to the final level!
Oh and there's also TF2 but everyone knows about that.