OpenSG is a scene graph system to create real-time graphics programs, e.g. for virtual reality applications.
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.
[1] Scene graphs are notoriously hard targets for multi-threading, as they contain very large data structures easily consuming hundreds of megabytes of memory.
Many scene graphs just lock individual nodes to prevent data corruption due to parallel writes, but that is only a partial solution.