Combat arms is pretty cool played that for a bit, but the greatest Free FPS i've ever played is Wolfenstien Enemy Territory. Its older but there is still alot of people playing it. Otherwise I play payed games, right now I am playing Counter Strike Global Offensive. By christmas Ill be playing Battlefield 4.
Well just looking them up i found out that team fortress 2 is now free to play forever, ive heard this game is bad to the bone, and i loved the first one.
Otherwise:
Here's a list of free First person shooters:
http://www.mmobomb.com/games/mmofps