(Note, there is another thread about this, but it's old, and it doesn't really have anything about the game, so I thought I'd post this to bring it to everyone's attention)
I just ran across this game yesterday.
Combat Arms. I just downloaded it today... I'll tell you what, this game is pretty dang awesome.
It's a fast paced FPS. You gain experience points (which allow you to unlock different things) and gold points (which allow you to buy stuff for your guns). You can also buy some other stuff with real money, which is how they make their money on the game. The game kinda has the same feel as Counter Strike: Source. But that's more because the guns are similar, and less because the game is similar.
It's pretty dang neat, check out the site, they have videos and stuff for you to check out. And I mean, might as well download it, it IS free. What have you to lose?
IntelCore2Duo@2.60GHz-4GB RAM-NVIDIA Quadro FX 570M-Windows Vista Business 32bit