0 AD is a free RTS game that has been in closed development for ages.
I've been monitoring its progress every so often, and today they have an announcement posted that they'll be going GPL (Possibly because it's become too big for a smaller team?).

Anyhow, link here: http://wildfiregames.com/0ad/

I really hope they get a good game out of this at some point, there's been a ton of work put into it.