From the about page:

Tremulous is a Free, OpenSource game that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans. Players on both teams are able to build working structures in-game like an RTS. These structures provide many functions, the most important being spawning. The designated builders must ensure there are spawn structures or other players will not be able to rejoin the game after death. Other structures provide automated base defense (to some degree), healing functions and much more...

Player advancement is different depending on which team you are on. As a human, players are rewarded with credits for each alien kill. These credits may be used to purchase new weapons and upgrades from the "Armoury". The alien team advances quite differently. Upon killing a human foe, the alien is able to evolve into a new class. The more kills gained the more powerful the classes available.

The overall objective behind Tremulous is to eliminate the opposing team. This is achieved by not only killing the opposing players but also removing their ability to respawn by destroying their spawn structures.

Tremulous (and the standalone server) is available as an Ubuntu Package, but I would suggest replacing the standard Tremulous binary with TJW's backport which adds some significant new features.

There are also builds for MicrosoftWindows and MacOSX.

BruceKingsbury has recently started two Tremulous game servers in Hamilton, and some of the regular players in New Zealand and Australia have formed a clan named Sudden Dretch. The name is a combination of the alien's lowest attacking class, the “dretch”, and a point in the game (“sudden death”, usually after 30 minutes) where nobody is allowed to build in order to push the game to a conclusion.