Hey don't forget megaglest or its derivative annex.
C&C twisted insurrection and C&C DTA are also free.
http://www.moddb.com/mods/twisted-insurrection/downloads/twisted-insurrection-public-beta-5-full http://www.moddb.com/mods/the-dawn-of-the-tiberium-age/downloads/the-dawn-of-the-tiberium-age-v11332
I'm surprised nobody here has mentioned MegaGlest. It's not that big, but it's a pretty fun open source RTS. I fell in love with it mostly because it was so easy to mod, and there was a wide selection of mods that completely changed the game.