-Company of Heroes 2 (surprise, surprise)
-Payday 2
-Spelunky
-The Binding of Isaac. (really cheap but super funny, and hours of fun (68h so far in steam) (no multiplayer)
-Terraria (really big free update in 9 days, perfect time to start. coop multiplayer)
Those are so good! You should give Don't Starve and FTL a try, if you haven't.
I tend to play lots of short games as well. Some recent-ish good ones are Guacamelee, Gone Home, Thomas Was Alone, Fez, Mark of the Ninja, Capsized, Rayman Origins, Hotline: Miami, and the Walking Dead.
I also boot up some oldies regularly, like Fallout 2, Jagged Alliance 2, Escape Velocity, ARMA 1, Sid Meier's Alpha Centauri, Half-Life, and Deus Ex.