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 Microsoft Windows, Linux, Solaris and Mac 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 was started, just like many other systems, at the end of the scenegraph extinction in 1999 when Microsoft and SGI's Fahrenheit graphics API project died. Given that there was no other scene graph system on the market nor on the horizon with the features the authors wanted, they decided to start their own.
OpenSG should not be confused with OpenSceneGraph which is entirely different scene graph API, somewhat similar to OpenGL Performer. Development on both started about the same time, and both chose similar names.