If you like big scale online fps' then Planetside 2 is pretty awesome and free to play (you can buy guns and stuff but after a few days I'm already unlocking things just from playing). If you've got steam look it up, if not check out http://www.planetside2.com/
The game is like a cross between Halo and Battlefield, think futuristic weapons and setting crossed with the huge scale Battlefield battles with tanks, planes etc. The maps are huge and the game supports an insane amount of players all at once. Really good fun for 5 mins or all day.