If you're looking for some quick action that you can join at any time and play for a few minutes or many hours, Continuum (subspace) is a great free one. You can play as a 1 man team against everyone, or get on a team. Either way can earn respect.
I've played since beta and I still get owned once in a while. Its very rough and hard for newbies.
http://www.subspacehq.com/