Goat Simulator is a hilarious game where you play as a goat and do lots of stupid things. The glitches are extremely funny to watch. Is cheap on sale.

Insurgency is an excellent online multiplayer tactical shooter which takes careful shooting to a whole new level. You can grab it cheaply on sale.

ORION: Prelude is an online multiplayer Halo style shooter with dinosaurs that costs only .99 cents. Great playing this with friends considering price.

Batman: Arkham City is an open world game where you get to play as Batman. The story is excellent, the fighting is thrilling and is definitely worth it.

Unturned is a free-to-play zombie survival shooter with a voxel art style. The gameplay is good and simplistic and is fun when played with friends.

While the story isn't very good and the voice acting is dodgy, the most fun in this game is just watching the whole world explode around you in HD.

Garry's Mod is an open world sandbox game with excellent mods available on the community workshop. There is nothing you can't create in this game.

Team Fortress 2 is a free to play FPS game that is great because it isn't pay to win still has an excellent trading community.

Portal 2 is an oldie, but a goodie. The 'portal' mechanics are great, the storyline is humorous and the co-op is great fun.

Tom Clancy's Rainbow Six Siege is an online tactical FPS game. Once you get into the multiplayer, it is a great fun but probably isn't worth full price.