Tremulous is a free, open source 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.

WWW: http://www.tremulous.net/