Combat Arms, a Free FPS is now on Steam! Combat Arms is a FREE online first-person shooter where players load up with dozens of different weapons including shotguns, machine guns, pistols, grenades, sniper rifles and much more all to frag the most enemies in multiplayer the least time. It is a fast-action multiplayer shooter with a huge library of modes, maps and weapons that will entertain players of all skill levels from new players to guilds of veterans.
Also Water Gun Wars which is a free FPS and open source free game maker is due for release in the next 1-2 months! The webpage is still in development but check it out at http://freefpsgame.com More info to come on this as it releases!