SpringRTS is an open source game engine that improved beyond TA and actually had UI features that Supreme Commander has drawn inspiration from.
The multilayer is epic, its totally LUA scriptable. People have re-purposed that engine for all manor of weird RTS type games. I sincerely hope this project is going to use that battle hardened, cross platform, actively developed, open source RTS engine rather than roll their own.
The engine launched back in '06 and has changed developers a bunch of times, which means the code is heavily rotted and it needs a lot of love. It has legacy issues and tooling issues out the wazoo, so it really needs some love from a charity or a google-summer-of-code-style project. The problem is they suck at the organizational stuff so they've never managed to put together anything like that.
The problem is that, even as the best-developed Spring game it still needs a lot of polish and it's never going to be more than a labour of love for a few hundred players and a handful of devs. Their donations just keep the server running and that's about it.
There appears to be a renewed interest, with people playing the original TA on and offline.
Here's a site with information about many of the various mods, remakes, etc: (http://www.tauniverse.com/)
And here's a sub-site with lots of information about online play (http://tazone.tauniverse.com/)
So far I think the best in the series was Supreme Commander: Forged Alliance (not Supreme Commander II).
I backed & am greatly looking forward to Planetary Annihilation (though not too excited about the cartoony art-work). Just made a donation to 0AD too.