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.
It's alright, but given the age it's been replaced by far better mods with zombies, but if you like playing as a zombie mauling survivors to death, this would be your type of mod.