I highly recommend this.
http://sourceforge.net/projects/warzone2100/?source=dlp

FPS:
- Team Fortress 2 (Available on Steam)
- Blacklight: Retribution (Available on Steam)
- Planetside 2 (Available on Steam)
- Tribes: Ascend (Available on Steam)
- Alliance of Valiant Arms (AVA)
- Warface (Open beta?)
- Battlefield Play4Free, Battlefield Heroes (Browser-based)
- Quake Live (Browser-based)
- Uberstrike (Download or play in Facebook) (Browser-based)
- Others

TPS:
- Warframe (Co-op, open beta) (Available on Steam)
- APB Reloaded (Available on Steam)
- Ghost Recon Online
- Etc.

MOBA:
- League of Legends
- Etc.

RPG:
- Star Wars: The Old Republic
- Lord of The Rings Online (Available on Steam)
- Spiral Knights (Co-op) (Available on Steam)
- DC Universe Online (Available on Steam)
- Adventure Quest Worlds (Browser-based)
- DragonFable, Adventure Quest, Other Artix Ent. games (Browser-based)

Strategy:
- Stronghold Kingdoms (Available on Steam)
- Age of Empires Online (Browser-based) (Available on Steam)
- Etc.

Simulation:
- War Rock (War Simulation)
- Star Conflict (Space Simulation)
- Etc.

Co-op:
- Warframe (Third-person Shooter) (Available on Steam)
- Spiral Knights (RPG) (Available on Steam)
- Etc.

Brawler/Fighting Game:
- GetAmped (Splash Fighter in USA)
- Lost Saga
Etc.

And many more exists and will come and had come.

Ooh, nostalgia, i used to play this lots when i was a kid. I support this recommendation!