Killing Floor. Just Killing Floor.
Oh, you want more? very well then. Killing Floor is an FPS available on steam where you kill zombies with things ranging from flamewthrowers to machine guns to grenade launchers. However, don't think you hold all the variety. As the massive Fleshpound tears you in half, you try to escape only to run smack into the invisible Stalker. So yeah. It's best played as multiplayer, though singleplayer is best for initial leveling up. Give it a shot, it's well worth the dosh.