Left 4 Dead is a cheesy (on purpose) horror-action game based in spirit on chase scenes from the movies 28 Days Later and 28 Weeks Later. Earth is overrun with super-fast zombies who sprint toward you and three other immune humans, as you shoot dozens within any given minute.
At times, you stand still and empty two entire machine gun rounds into 20 or more zombies zooming into your face. That frenetic pace carries on through a hospital, an airport, city buildings and streets.
Other reviewers love this game, partially for the horror-reverential, cinematic scenes and dialogue ("Fire in the hole!"), but mostly for the zippy action, especially online, where you can play through the narrative with one to three other gamers in tow.
Personally, I grew weary of "Left 4 Dead" after an hour. It's something of a sleek one-trick pony. Run, stop, shoot. Wash, rinse, repeat.