Get counter strike source
It's all the fps multiplayer frag festing you need with the added bonus of zombie escape.
I keep playing other games (medal of honor beta, bad company 2, fuel of war, call of duty blah), but in the end they all suck and I return to CSS no stupid leveling system, vast numbers of servers, huge number of mods and maps.
But all these other games act as little excursions to remind me why CSS is good. Sure they look good but at the end of the day they suck.