Zombie Panic! Source
ZPS Is great Free2Play game...
When i created steam, i bought Garry´s Mod, and then, i was searching for some F2P Game, then, i saw ZPS.
I installed it, and it was great! Let me tell you why.
ZPS is Zombie FPS F2P Action Game
Its only MP, not SP.
The main task is... You must survive for 8 minutes on some maps, or make some objectives(other types of map) I think that there are only 2 types of game, survival and objectives.
In game, u can play as zombie, so u must hunt for the survivors, and if u are survivor, u must barricade your survival place, search for ammo&guns.
If u cant survive for 8 minutes, and u die, u will turn into zombie, and hunt your previous team mates!
Also, in game is need to make some team and communicate with eachother, because if u are playing solo... u cant do it.
Great f2p game
Try it! U can download it for steam for free, give it few tries, and maybe, it will be your next favorite game!
btw. sorry for some grammar fails, im not from USA/UK, but from CZE :)