0 A.D. A free, open-source game of ancient warfare
play0ad.com
play0ad.com
Alpha is an understatement IMO as this game has great cross-platform support, a multi-player lobby, pretty balanced gameplay and amazing graphics, plus a ton of other features too numerous to mention.
There's also a huge scripting component in JavaScript, so it's pretty easy for most devs to contribute maps and make gameplay PRs.
There was also a long delay between v23 and v24, but now the team seems back on track and committed to a 6 month release cycle.
To play: https://play0ad.com/
To contribute: https://trac.wildfiregames.com/
EDIT: No really, why? Is there some bigger event I missed?
https://news.ycombinator.com/item?id=28201265 Beyond All Reason: Open-source RTS reimagining Total Annihilation
I'm not sure about the other two.