It came out even before DayZ got all the glory. I used to have fun playing Dynamic zombie sandbox on Arma 2, Now there is a new version for Arma 3 many things still need to be polished but it's looking really nice and people won't be attached to servers or anything like that. It can be played LAN(sort of single player) or multiplayer.

Here is their website.
http://dynamiczombiesandbox.com/

A gameplay video.

I tried it a bit and found the damage lacking. It took 3-4 head shots with just about every weapon to kill a zombie.

I noticed that too. There is a new version (0.5) that improves it and now head shots means dead.
It's a relatively early version so I guess the mod will improve over time, and since I don't play multiplayer too much, I find this mod a better option than DayZ.

Tried it the other day. Thought it was awful to be blunt. Got surrounded by a horde of "zombies" that I couldn't kill. I think they could move as fast as me or faster and were nearly invincible.

(note I haven't played the updated version, but it might be worth checking out)