Goat Simulator is just, hilarious. Don't expect a true game when buying this, just invite friends over, and watch the madness ensue. Give it a chance.

Less well-known, but a great tactical war-game that's easy on the processor. Robust, simple to learn gameplay, with fair multiplayer. Not bad, & cheap.

Chivalry is a mainly melee-based game with elements that are easily comparable to an average FPS. Great fun, great fights, great buy.

Broforce, as the name implies, is probably going to be more interesting to men, as it includes testosterone fueled violence & war. Still worth the cash.

Dungeon Defenders is a Tower-Defense game at heart, but has options to simply run and gun too. This game is suitable for many playstyles. Give it a try.

Garry's Mod is truly an amazing game. You have so much power and freedom in this sandbox game, that almost anything is possible. Play this with friends.

Civilization V takes the cake in the strategy genre. Playing alone & with friends, this game is well-done. Wait for the sale on the Collector's Edition.

Team Fortress 2 is a multiplayer game that puts a neat spin on the classic first person shooter genre. Really fun, really addictive, really cheap.