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. 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.
Pretty fun game! Only downside I see is that vanilla tremulous doesn't get updated. You have to manually search for patches and mods scattered around forums. Community is very nice and helpfull though :)
"Tremulous started development before Natural Selection, and both games share a common ancestry with Gloom, a Quake 2 mod." -Tremulous Wiki
Tremulous is a great game, IF you get a good server. the gameplay is fantastic and the graphics brilliant for an Indie game. the weapons and Alien species are varied and interesting, but none of that matters if you don't get a good server. good server and you're set for a half a day of good ol' fashion time-wasting.