Slaying hordes of zombies is always fun, the fun multiplies when you have a group of people with you to mow down the endless waves of the sprinting undead. Valve have yet again come up with another multiplayer game that is capturing peoples attention, this time with more team work and less PVP.
Left 4 Dead puts you and a team of up to 3 others in scenarios where you must battle your way through zombie infested locations in order reach a safe zone. Why these put themselves in these locations is a different story. All we know is that the zombie slaying action is amazing.
Since Valve are unable to make a third game of anything, it could be quite some time, if ever, before we see a new entry to the series. For now, we must find other games like Left 4 dead. Here are some suggestions.
Borderlands 2
Dead Island
Resistance 2

There was a brilliant and mostly forgotten game coop game mode in Resistance 2. Not sure how active the community is at this stage, but it likely still supports friend invites. Think it was a 4 player coop mode and it was high paced madness at times. Swarms of enemies and you were just churning through them with all of the mad weapons available in this game. I'd say a Resistance remaster is gonna be released at some stage, so it might be a good chance to play it again if they do release it.