Zombie Panic! Source is a standalone cooperative, survival-horror, first-person-shooter! Set in the middle of a zombie apocalypse, players start as a small group of survivors attempting to stay alive. Each map will have its own objectives to complete and win the round or survive by a period of time.
Great mod. Love the fact that you are playing against live zombies instead of AI zombies. The weapon/ammo weight plus exhaustion adds to the realism and makes being a team player a necessity.
One of the few games that enforce team play for success.
Downside is there are few updates for some time although a new update is rumored to be around the corner.