According to this reddit post, Hi-Rez Studios, the developer of a free to play first-person shooter Tribes: Ascend and owner of the intellectual property rights to previous games in the series, decided to release most of the older Earthsiege and Tribes games as freeware.
Earthsiege games are oldschool mech simulators, not unlike Mechwarrior: players control giant robots during the battles and customize them outside of it. Tribes titles are set in the same universe but their gameplay is more about fast-paced, skill-based multiplayer first-person shooting.
Keep in mind that none of the games have been modified to run on modern operating systems. This means that Earthsiege might require the use of DOSBox while Tribes: Aerial Assault must be played through a PlayStation 2 emulator like PCSX2.
Interestingly, the games seem to have been made freeware as far back as in late August. For some reason, they’ve managed to slip under everyone’s radar for more than two months.