Although not a first person shooter like Left 4 Dead, I would say
Alien Swarm: Reactive Drop captures the co-op gameplay nicely, but from a top down perspective. Try to reach the end of a map, while doing objectives inside the level and holding off the enemies. It's free to play, so try it out with some friends on PC.
For the PC players of Left 4 Dead 2 who haven't done any custom campaigns yet, u guys have missed out on two of the most amazing campaigns the game has to offer. These campaigns contain lots of surprising elements and challenges and has some really original levels with a kind of Portal feel to it. Try them out if u feel the itch to play Left 4 Dead 2 again. Just need to subscribe to the Steam Workshop and it will automatically download the campaign for u to play in the game.
Campaign 1:
Questionable Ethics
Campaign 2:
Questionable Ethics: Alpha Test
It is recommended to play the campaigns in order and with friends. :>