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.
This mod has pretty good weapons and the map design is alright but it really just doesnt feel like nazi zombies. The mod has bonuses like max ammo or insta kill and you can buy weapons off the walls and open doors but it just doesnt feel the same. Also the zombies are the fast zombies from half life 2 and that is the complete opposite of the slow zombies from nazi zombies.
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.