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Aftermath is a hl2 mod which involves the player killing waves of the undead with your friends or alone using weapons, your knife or even blow them up with grenades and set them alight with Molotov's.
It says what it has to offer in the first paragraph of the main page.
It's a copy of COD zombies on source, and to be honest, it's nicely done.
While some bugs exist, it shouldn't stop people from indulging in mindless zombie killing. For awhile, anyway. The mod quickly gets rather old after a couple of hours.
Regardless, those who liked COD zombies should expect to have some fun. Those looking for more may need to look elsewhere.