OpenSceneGraph
The OpenSceneGraph is an open source high peformance 3D graphics toolkit, used by application developers in fields such as visual simulation, games, virtual reality, scientific visualization and modelling. Written entirely in Standard C++ and OpenGL it runs on all Windows platforms, OSX, GNU/Linux, IRIX, Solaris and FreeBSD operating systems.
Original idea was to use Ogre in real-time visualisation in the Seminaarinmäki project, but the Ogre's lack of scenegraph concept made OpenSceneGraph more viable solution for us.