Some of you may know of the interesting(good and bad) Military SIM Arma and Arma2. Today I found quite possibly the most ambitious mod for, I'd like to say, any game this decade. Basically a persistent, sever based, PVPVE, survival zombie game. The map is from what I have read 215 KM^2 Huge.
The easier way to explain it is to show a Video(coarse-ish language and gets into it at about 4 mins if you hate intros).