Coin3D

Coin3D is a high-level, retained-mode toolkit for effective 3D graphics
development.  It is API compatible with Open Inventor 2.1.

Open Inventor, for those who are not familiar with it, is an OpenGL-based,
retain-mode, 3D model scene-graph rendering and interaction library, which
has become the de-facto standard graphics library for 3D visualization and
visual simulation software in the scientific and engineering community.