Planetside
Planetside blends First-Person Shooter with the massive land / area's of an MMO to form a new genre "MMOFPS", Planetside is by far one of the best MMOFPS's around, it pit's three empires (Terran, Vanu, NC) against each other to fight on a daily basis for land control. The game has continued to be supported by SoE Throughout it's life and recently had a server merge to increase the amount of players for even larger battles!
The game offers something for everyone, you can run around hacking Enemy bases, Attack on Foot, Attack from The air, Attack on the ground in one of the many vehicles, and even attack in BFR's (giant ass robots). This game really is the ultimate First-Person Shooter experience for MMO players and serious FPS players alike.