Probably the best sandbox game ever, and is a good game to play with friends. However, you need Counter-Strike: Source to play most multiplayer modes.

Really fun free MOBA, but expect some trolls and lots of insults coming towards you. Also Russians.

Gory First-Person zombie shooter, it's really fun to play with friends. And it's where my username comes from.

Fun game, only thing you pay for are cosmetic things like hats.