ZP:S is a HL2 mod. There are 2 sides, humans and zombies. 20 players a game. At the start of each round there are 3 alpha zombies and 17 humans. The humans ravage the map for any weapons and tools they can find. Drawback is you can't be a hog because the more shit you carry the slower you run and walk. The humans basically have to kill all the zombies or survive for a certain amount of time.
The site: http://zombiepanic.org
Some videos..
Being a Zombie in Zombie Panic Source Part1
Zombie Panic: Source fun
Zombie Panic! Source Gameplay Part 1
Zombie Panic! Source. Ravenholm Overrun
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