This series is zombie slaying at it's finest.
http://en.wikipedia.org/wiki/Left_4_Dead
This series is non-stop action from the FPS perspective. The mechanics are similar to Half Life and Counter Strike. If you're not simply naturally terrified of zombies, the scares come from the overwhelming number of fast-type zombies as well as special infected. You often find yourself against seemingly endless hoardes of zombies. In some situations, the zombies are in fact endless until you fight your way to a trigger that ends the hoardes.
Co-op is a main focus in the series, allowing up to four players to play together at a time. While it is unfortunate that you will usually experience annoying kids running around like idiots and doing their own thing, if you put together a decent team the co-op is amazing. I found that even the split screen co-op was great (on a big screen).
Left 4 Dead 2 was actually a better game in my opinion. This is rare in this genre.
http://en.wikipedia.org/wiki/Left_4_Dead_2
L4D3 is on it's way.
I highly recommend this series. This isn't exactly a scarefest, but it is a genuinely brutal, over-the-top depiction of zombie survival as it's finest.
