Open Scene Graph is a pretty good, very well designed cross-platform 3D engine. Contrary to Ogre3D, for example, it does not provide "game engine" features, and concentrates on being a very nice abstraction on top of OpenGL.

• It is quite lightweight, and does not force a framework on you: you can use as little or as much of it as you want, and use it through SDL, SFML, wxWidgets, QT...
• It is a great learning experience: as you learn the library, you understand more and more about the underlying OpenGL and the way it has been designed
• Pretty much ready to use: it has loaders for mainstream 3D formats
• Shaders friendly

Have a look at the extensive list of samples.