Tribes Ascend is a multi player FPS. it is said to be free when it is officially launched.
looks fun, give it a try.
and as mentioned by DJ TTK, you can try Red Orchestra 2 which is said to be a combination of INS and DOD:S. or you can try Star Wars : The Old Republic which is a MMORPG or lastly but not the least you can check out ArmA2 : Operation Arrowhead which is a tactical shooter/military simulation.