Tremulous
From the official About page "Tremulous is a free, open source game that blends a team based FPS with elements of an RTS."
Installation
Tremulous can be installed by installing the package "tremulous". It can also be installed from Add/Remove by selecting All Available Applications and searching for tremulous.
Backport
Many servers support, and request the installation of, an upgraded version of the tremulous client dubbed "the backport". This client includes back-ported updates from the, as yet, unreleased next version of the game. Typically, the backport is not required to play fully, but is required to obtain administrative privileges in the game.
For easy installation, here is a script that will backup your tremulous file as tremulous_old, and install the backport:
cd ~ && wget http://tremulous.tjw.org/backport/linux/tremulous.x86 && chmod +x tremulous.x86 && sudo mv /usr/lib/tremulous/tremulous /usr/lib/tremulous/tremulous_old && sudo mv tremulous.x86 /usr/lib/tremulous/tremulous && sudo chown root:root /usr/lib/tremulous/tremulous
Links
https://help.ubuntu.com/community/Games?highlight=%28tremulous%29#Tremulous