Counter-Strike Source

Yet another amazing game. I paid for this... Twice. It uses the source engine, so that means easy mods, easy control and customisation of a server, and all the variants of the source games you could want:
Zombie Infection/Escape
Deathmatch games
RPG levelling between games (Increase health, regen, ammo, etc)
Surfing (personal favourite)
Deathrun (run past various obstacles, another favourite)

http://store.steampowered.com/app/240/
Link to buy it on source

Example picture: