Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Spring is a versatile 3D RTS game engine. Using extensively Lua for scripting game-specific code to make nearly every aspect of the engine customizable, from GUI, to unit AI, to pathfinding.
GWIC (Gynoid World Interactive Constructor) is a sandbox environment where you can freely create, edit and play with complex worlds contains almost unlimited terrain space, static, dynamic and physics objects, and unlimited count of actors, like a script-driven physical gynoid models. Or anything you can imagine!
PointClickPress is a web app game engine that allows for the creation of point and click games & interactive stories. Currently releasing builds every 2-3 months. Playable demo: www.pointclickpress.org. For questions pls visit the forum.