I'm currently playing Borderlands 2 as a mechromancer, on PC.
Borderlands supports up to four players, and the difficulty curve goes up (so you level faster), therefore I've been playing it with friends. I have to admit, I do get bored of it after about eight hours, but I do love my giant mech. It's a main stream game, so I'm not gonna go on about it.
The important game on this list is “The Killing Floor,” we all know zombies need to die and this is just hordes of zombies. Unlike L4D, it's actually fun and requires strategy. You can play as one of many classes, and if you play this we should be playing it together.
Let's see installed PC games include: Doom 3: BFG Edition, Half-Life 2: Deathmatch, Hard Reset, Killing Floor, Metro 2033, Quantum Conundrum, RAGE, Ace of Spades, Zombie Panic Source, Batman: Arkham City GOTY, Skyrim, Saints Row: The Third, Sleeping Dogs, Dead Pixels, Max Payne 3, Spec Ops: The Line, and XCOM-Enemy Unknown.
Current PS3 games include: Tokyo Jungle, Wipeout HD, Metal Gear Solid HD Collection, and Red Dead Redemption.
Current 360 games include: Halo 4, and Halo Reach.
Also Pokemon Black & White 2.