Day of Defeat
Day of Defeat (DoD for short) is a game where players enlists in an intense brand of Axis vs. Allied team play set in the WWII European Theatre of Operations. Players assume the role of light/assault/heavy infantry, sniper or machine-gunner class, each with a unique arsenal of historical weaponry at their disposal; missions are based on key historical operations; and as war rages on, players must work together with their squad to accomplish a variety of mission-specific objectives.
Like Counter-Strike, Day of Defeat started out as an unofficial 
GoldSrc mod before being picked up by Valve for a commercial release. It later received a 
Source remake entitled 
Day of Defeat: Source (DoD:S for short), which added enhancements such as HDR lighting, physics, new weapons, and updated versions of classic maps.