Wolfenstein:Enemy Territory and Quake Wars:Enemy Territory.
Wolf:ET was released by Splash Damage, for free when Activision pulled the plug on it, in 2004. It is still amongst the top 3 most played fps on the pc.
Quake Wars was released in 2007 and is played competitively. It has a demo.
Their games are more about cognition than twitch skills (unlike, say, your CoDs and Counter Strikes) which makes every match you play, unique. The meta-game of these is what has ensured their longevity. Believe me, the CoDs and Halos are nothing compared to their games.