Spring is open source and free, fair size community (around 250+ players online at peak times).
It's still being developed (mainly bugfixes),although the graphics are not cutting edge, it doesn't look too bad. CA has slightly better effects than BA, but it's nowhere near the eye candy of supcom. It does run really smooth though, and someone is even developing a multi threaded version (currently BETA).
It's very practical though, can zoom right out and see the whole map on screen, or zoom in really far, can go first person and control units too. There's even a widget so spectators can follow the camera movement of another player, and another widget to show ally mouse/cursor positions.
If you are new to it, best to play against some bots first, or watch some replays, or spectate some games. Really steep learning curve.
Mod info here
http://modinfo.adune.nl/index.php?MOD=ba685 if you want to get technical.
Replays here:
http://replays.adune.nl/
It has pretty good peripheral programs, Spring downloader will autodownload maps, mods, widgets and has built in torrent system if the mirrors are down. It also integrates with Teamspeak to autoassign teams to their own channel.
There's two lobby clients, TASClient and Springlobby, SL is newer and not as stable/well developed yet, but works on Linux, IMO TASclient is easier and faster.
Same guy that wrote SD also made an autohost program that will host games all day, players just use !help !balance !start !votemap etc, so there's no host to ragequit or lag out. (There's also a similar program written in Perl that does pretty much the same thing).
Graphics can be buggy if you are using an ATI card and could be hit or miss with drivers, but once it works it's usually pretty stable.
Just got some screenshots (clicky for large)
Can zoom right in to the commander name badges.
