Tremulous

Tremulous is a first-person shooter crossed with real-time strategy that is very similar to the Half-Life mod, Natural Selection and rings a bell of Starcraft. The game generally involves choosing some sort of unit and trying to capture the other team's base, however one can spawn as a worker unit and build structures to defend the base.

There are two factions in Tremulous: Marines and Aliens. The marines are humans who fight with guns and similar weapons and the aliens fight using their claws and other biological weapons.

There is no single player to the game, one can only play multiplayer on a network or on the Internet. There are several servers that host Tremulous games online where one can find people playing at any time of the day.

Main Page: http://www.tremulous.net/
Download: Download Page
Ubuntu: Available through Synaptic.