The Tribes universe has been around for more than 20 years (sinc Earthsiege in 1994) and current owners Hi-Rez Studios (Tribes: Ascend) are celebrating this fact by giving away all of the games as free downloads.
The various games in the series/franchise/universe have been developed by numerous studios including Dynamix and Irrational Games.
Earthsiege, Earthsiege 2, Starsiege: Tribes, Tribes 2, Tribes: Aerial Assault, Tribes Vengeance, and Tribes Ascend are free, so what are you waiting for?
The games are available to download via the Tribes Universe website.