Hi-Rez Studios, the developer behind the Earthsiege and Tribes games, have just put up all legacy titles for free. The studio included this message:

Welcome! Join Hi-Rez Studios in celebrating over twenty years of the TRIBES Universe! Below you will find links to download the legacy games in the series. We love these games and hope to let you share the experience of the World's Fastest Shooter.

The games available to download now are Earthsiege, Earthsiege 2, Starsiege: Tribes, Tribes 2, Tribes: Aerial Assault, Tribes Vengeance and Tribes Ascend.

I can only personally vouch for the quality of Tribes Ascend, but that's a pretty fantastic game and this is an incredibly generous gesture by the studio, so it's worth checking out the free games on offer. You can find download links and keys on Hi-Rez Studios's website.