The game is called: Tremulous®, it is a GNU released game (Freeware):
This was a mod for Quake3: Arena, but then became a Total-conversion/Source-port to the Ioquake3 engine so that it could be a stand-alone game.
The Game features two teams (Aliens and Humans), now the Human's have to kill the Alien's and get money to buy guns whilst the Alien's have to kill the Human's to get 'Evo's' (Which is used to evolve into other types of aliens).
The Alien team start off with players that choose from Grangers (or builders) to a Dretch (Who kill and get evo's), now there are quite a few classes of these types and the most hardest to evolve to would be the Tyrant (costs 5 Evo's) and is almost impossible to kill.
I can't add more detail to it, but I can share with you the Homepage and Wiki about it:
Homepage //
Wiki
Disclaimer: I don't know if you have heard of this game or not, it came out in 2006 but has a recent Gameplay preview for an upcoming release (Or GPP 1.2). And yes, I do play this.
![Tongue](images/smilies/d_tongue.gif)