I know that Team Fortress 2 is quite popular among many of you - as was MW3 during the free weekend, so I'm recommending Blacklight: Retribution.
Blacklight Retribution is a Sci-FI MMOFPS consisting of quick intense PvP battles between two teams. As the match progresses, points unlock similar to a MOBA system that allow combatants to purchase stronger boosts in both armor and weaponry. Combine this with your ability to custom build various guns in your portfolio and Blacklight Retribution becomes very engrossing very quickly.
Features:
Weapon Customization: Customize everything about your weapon from barrel, stock, muzzle, optics, magazine, ammo type, colors, and even weapon tags for that extra performance edge.
Custom Classes: Define your own play style based on skills, equipment, and armor.
Futuristic Weapons: Use unreal weaponry like HRV goggles (see through walls), automatic sniper rifles, and mechanized fighting suits. It just entered open Beta yesterday, so be one of the first to try it out! Now or at release, it will be Free!
http://blacklight.perfectworld.com/