SW: Empire at War / Forces of corruption - Modded
Mods for EAW / FOC - Phoenix Rising / Republic At War
GTAIV is still fun to go lolz about in.
Resident Evil 5 is fun if you have a friend to play with.
F2P - FPS
Despite the naysayers, I play Combat Arms. I got a Major 5, Legit 2.26 kdr, and perm emod, so if you want to level up in a legit room, then add dTereusb
Borderlands - technically with a google search you can obtain it and the DLC free.
If you really want to rack your brain for games to play: Gameranger.com
Most of our clan plays games on that list, in multiplayer, and its easier then hamachi.
Also, Everquest II and STO are free to play now too.