Spring is a free, open source, fully 3D, real-time strategy game engine. Originally designed to run content from Total Annihilation. Since it's early days, the engine has matured significantly. While retaining some support for units in TA formats, games for the Spring Engine can now be developed using more modern technologies. Using the Lua scripting language to allow everything from extending the GUI and Gameplay to Unit Animation, allowing units to be created in modern 3D model formats and an extensible AI framework all while having an open licencing model (GNU GPL), the Spring Engine is an excellent choice for any RTS Game Developer. Many different projects covering a wide range of popular RTS genres and diverse themes have already been built for the Spring Engine establishing it as a key contributor to the future success of open source gaming and free to play games.
A vid of me playing in survival mode (normal difficulty) showing off the gameplay a bit. Survival mode can be played either by yourself or with friends online in co-op, teams or even free for all.