ArmA 2 Free Realistic Action FPS

I've stumbled across this cool first person shooter: ArmA 2: Free. It's in my opinion the most realistic war simulator game that's also free. The free version lacks a local singleplayer campaign, but there's a lot of online co-op and team vs team servers. It features a wide array of weapons, vehicles, and a semi-destructible environment. You can destroy buildings and vehicles, even make craters in the ground from artillery, but of course there's limits. The downside to this game is that there's no permanent ranking or level system. Individual servers may implement their own ranking system, but scores are usually wiped every server restart.