Another fantastic free RTS built on the Spring engine. It features terrain deformation (you can literally build a bridge or flatten a mountain) and a huge variety of units. Check out Zero-K on Steam.
Download hundreds of community-made maps from TA Universe . Total Annihilation Free Download
This is the most popular mod, adding Tier 3 and Tier 4 units that weren't in the original game. Find it at TA Escalation . Another fantastic free RTS built on the Spring engine