Nothing beats Halo xbox LAN action (until Halo 2, but I digress). Oooh that or playing Battlefied 1942, everyone should give the demo a try, it has the best level in the game and there are still 50+ servers for it all the time. Also, America's Army is an extremely solid FPS simulation (
http://www.americasarmy.com/) and is free because it's a recruitment tool.