Hi-Rez Studios in celebrating over twenty years of the TRIBES series by making them available free to download. The development team is focusing on Smite which has turned out to be a golden goose, and thanks to this fact, they have decided to continue to patch and support for the game. Hopefully we can all hear people shazbotting again.
Title
Earthsiege
Earthsiege 2
Starseige: Tribes
Tribes 2
Tribes: Aerial Assault
Tribes Vengeance
Tribes Ascend 		Release Date
1994
1996
1998
2001
2002
2004
2012

Most of the games are in ISO format, which is a commonly used disc image format, and you can burn the ISO on to a disc, or just use mount software, to install the games. Some of the older games might require DOSBox as well. If you have any compatibility issues, just head on over to the Tribes subreddit post.

Links: Official Announcement