Coin3D is a cross-platform , a high-level, retained-mode toolkit for effective 3D graphics development. Also it’s offer a comprehensive and object-oriented 3D toolkit solution for interactive graphics programming problems. It based on Open Inventor. Coin3D develop by Kongsberg SIM-(Norwegian company).
It really have impressive scene database that make 3D programming more easier. That the part I love 
. Also it come with lot of examples that that explain most of it features nicely and clearly.
So if you want to develop a 3D application (e.g. visualization ) with that can be easily port to many platform, then Coin3D is your choice. It support many platform also many GUI ( widgets ) system ( Qt , Windows, Mac OS X and Motif/X11 ). Also it have more features that I’ll highlight it later.
Now I’m using Coin3D with Qt4 to develop a visualization system that run under Linux/Windows.