those you have to pay for/ not multiplayer.
some free 3D online FPS's are;
Nexuiz
Paintball 2.0 (very very fun)
Cube 2:sauerbraten. you can edit maps, in game.
Tremulous. aliens VS. humans.FPS.
You can make your own FPS with FPS creator. (search it on google. your can get a free version, but the pay version comes with more stuff. in the pay version, you can make online maps, then give em' to your friends and play them online.W0ot).
World of padman. your like a cartoon alien/bug and you kill other alien/bug people.
Assault cube. (very very fun)
Celestial impact. one of your weapons include a Ground-crafting gun which you then fight upon.
Tell me when your do going through all of those. :P