Wikipedia
OpenSG
OpenSG is a scene graph system to create real-time graphics programs, e.g. for virtual reality applications. It is developed following Open Source principles, LGPL licensed, and can be used freely. It runs on Windows, Linux, Solaris and OS X and is based on OpenGL.
Its main features are advanced multithreading and clustering support (with sort-first and sort-last rendering, amongst other techniques), although it is perfectly usable in a single-threaded single-system application as well.
It is not part of Khronos Group.