During the past 2 years I've gathered all of the online FPS games I could find and put it into one list, also got a lot of help from other YouTube users.
Search "TONS of free online FPS games for you to play! [With DL and Gameplay links!]" in the YouTube search and you'll find the video, it's got both Download links and links to gameplay videos, each and every one of them.
Hope you like it!
1. Battlefield Play4Free
2. Team Fortress 2 - Play4Free
3. A.V.A. Online [Alliance of Valiant Arms]
4. Americas Army
5. F.E.A.R. Combat Online [First Encounter Assault Recon]
6. Assault Cube
7. Blackshot
8. Combat Arms
9. Crossfire
10. Operation7
11. Rising Eagle
12. Soldier Front
13. Sudden Attack
14. True Combat: Elite (An Enemy Territory Mod)
15. Urban Terror
16. Warrock
17. Enemys Territory: Return to Castle Wolfenstein
18. WolfTeam
19. Nexuiz
20. Quake live
21. Control Monger
22. D-Day: Normandy
23. Dispatch Of Army
24. Fallen Empire: Legions
25. Kkrieger
26. Kuma/War Get
27. Realm Wars
28. Sting Online
29. Tremulous
30. Tribes
31. K.O.S. - Secret Operations
32. OpenArena
33. Crazy Shooter Online
34. Genesis A.D
35. Uberstrike
36. Smokin' Guns
37. Mercenary Wars
38. Project Blackout
39. M.A.T. - Mission Against Terror
40. Alien Arena
41. Digital Paint: Paintball 2.0
42. Purge Online
43. First Encounter [Czech!]
44: Upcoming game!: MKZ (NOT AVAILABLE YET!!! Check out their homepage and sign up for updates!)