Zombie Survival
Zombie Survival is a mod coded in the LUA scripting language for GMod. The good thing about the GMod mods, is that you dont actually have to download anything to play on servers running them. Its just a simple matter of connecting to the server and your game client will do the rest.
If you have ever played Counter Strike Source's Zombie Horde, then you will probably understand the basics of this mod, although Zombie Survival takes the limitations of the Source engine to the max and with the help from LUA adds a lot more game play and features to the game. The operative word in this mod is Survival, and thats just it, once you are killed as a human, you will become a zombie and your role changes from being a zombie hunter, to being a 'brain, flesh eating' hunter. One of the interesting things about this mod is the possibility to continuously playing and respawning if you die as a Zombie and the only way the round will actually end, is when the time limit is reached or all of the humans are 'turned' in to a Zombie.

The additional scope in this mod is the way that it creates roles for the zombies depending on how you have performed throughout the round. It makes it especially more enjoyable to play as a zombie because you can quite literally evolve, from the lesser state of a slow, sluggish zombie into a fast 'chemical' distributing acidic fiend that poisons the humans upon contact.