Hi-Rez has made all the Tribes games available for free, as well as the Earthsiege series that predated it. They're all PC titles, with the exception of Tribes: Aerial Assault, which I think might be the first PS2 game released as freeware by its developer. (It comes as an .iso file, so you'll be able to play it in an emulator and on chipped/modded PS2s.)
I'm not sure how active or functional the multiplayer is for the previous Tribeses, but there's some single-player stuff in there, and I'd imagine it would be quite fun to download a few and take a nostalgic jet-pack-powered trip down memory lane. I've not played a Tribes game before, as you may have gathered, but the first-person, mech-based Earthsiege games have caught my eye here.
Here's the full list of newly free stuff (Tribes Ascend, the most recent in the series, was of course already free-to-play):
- Earthsiege 2
- Starsiege: Tribes
- Tribes 2
- Tribes: Aerial Assault
- Tribes Vengeance
- Tribes Ascend
After a period of absence, Hi-Rez is now working on Tribes Ascend again.