Unreal Tournament. When i was 14 I played the demo and ended up competing online and becoming one of the top 3 1v1 duelers in the country. I placed 3rd at a National qualifier for the 2001 World Cyber Games in LA, and got flown to South Korea to compete against an international player pool for a 30,000$ first place prize. That time is over now, in college and all, but I still love that game. Counter-strike is good too.
www.gotfrag.com
www.esreality.com
www.cached.net
www.thecpl.com
Gaming is going professional, check it out.