Combat Arms is a totally FREE to download and FREE to play MMO. It is a FPS (first person shooter) Here's a video of what the game play is like:
The company that put it out NEXON gives the game away for free and you get your chose of 6 basic caracters to play from, even female characters (ROCK!!) and you get the basic weapons (M16 Rifle, 9mm pistol, frag granades and a knife. Which are all good for game use. When you enter the game you earn XP but also GP which is game money and you can upgrade weapons and gear, but the upgrades are temporary, lasting anythign from 24 hours up to 90 days, depend on how much GP you earn and want to spend.
The way Nexon makes their money is they have a Black Market where you can spend real money on upgrades. You go to Blockbuster or Target or Walgreens etc and buy their game card and it has X number of game money and it's THAT money you spend on weapons or gear like a gold AK-47 or different looking characters than the 6 basic ones, and THAT is how they make money and why the game is free.