3D Online Ego Shooter (Open Source & portable)

Open Source
Warsow
http://www.warsow.net

Urban Terror
http://www.urbanterror.info

Assault Cube
http://assault.cubers.net/

Alien Arena
http://icculus.org/alienarena/rpa/

Nexuiz
http://alientrap.org/nexuiz/

Open Arena
http://openarena.ws/media.html

Sauerbraten
http://sauerbraten.org/

Tremolous
http://tremulous.net//

Smoking Guns
http://sourceforge.net/projects/smokinguns/


Freewware
Wolfenstein Enemy Territory
http://wolfenstein-city.4players.de/wet-data/vollversion/

S.W.I.N.E.
http://www.gamershell.com/download_11942.shtml

Thank you very much for the links Einherier. I used to play Alien Arena while it was in beta. Brings back sweet memories

Here is another good Freeware MMOFPS that I came across

Alliance of Valliant Arms
http://ava.ijji.com/

I know that it cannot be made portable. But I am just adding it to your great list.

Here is another great one:

World of Padman - http://padworld.myexp.de/

200 free 3d online games (some opensource & portable) you will find on my homepage:
http://einherier.blogspot.com
many are suitable for liberkey.

Most games you can copy the installation folder to your usb stick and it works.