I forgot to mention (sorry!) that Savage: The Battle for Newerth is now officially freeware. I read this a while ago but at the time I couldn't find links to where to actually download it. These days it's listed as a free download on the official website - at the time I came across this it seemed more conjecture than reality that it was freeware.
Savage is one of these hybrid RTS/FPS games that seem to be cropping up more regularly these days e.g. Tremulous. It's got great graphics and is free (I would attach a screenshot but I broke Firefox!) so, well, why not!? Still, I wish they would make games open source rather than just abandoning them as freeware so at least the community can fix any problems. I guess S2Games are probably a little scared of competition for the upcoming Savage 2.
Speaking of formerly commercial games being open sourced, Battle Chess is apparently now open source freeware! It would be an interest project to port it to modern operating systems. Thanks go to benjamin for pointing this out in a comment on a previous post. However I'm wondering whether sadly it is a mistake though because although the page says it is GPL there is no link to any source - there's none in the linked zip file.
Edit - thanks nelipa for point out Battle Chess is only freeware; liberated games is telling a fib.