L4D, quite old now (sort of) good if your playing with friends, sh!t if not with them. Its not a game you can play over and over, but is great fun when your in the heat of the moment 
Old but then so is Counter Strike. Replayable just like Counter Strike.
Its a great game, it just works, its very well balanced, AI "director" mixes things up every time. Versus mode is very good with some buddies, you can play as special infected zombies and wipe out the survivors
