http://www.enemy-territory.com/
Just started playing this week with my squad, we're a big bunch o' n00bs but its a great game. I think there are few true team based games around and this is one of 'em.
I always thought it was a MOD for RTCW i.e. need RTCW to play, but the whole deal if free
Looks kinda dated these days, but there is a huge bag full of playablility that most nice looking games tend to forget about + you dont really get time to checkout how nice the trees or sky looks etc.
Totally free, team based, fast paced, punk buster supported, fps. It doesn't get much better than that
*Note to NovaLogic
FREE stand alone MOD