QBlock is a cross-platform rapid-prototyping game design engine written in C++, SDL and OpenGL. Features game asset management, scrolling tilemaps, particle systems, 3D model support, Lua scripting and more.
PsychoEngine is an Open-Source WYSIWYG 3Dgameengine/creation utility, that hopes to eventually compete with other Open-Source game engines/creation tools as well as commercial engines and tools, both visually and in playablity.