Easy to use cross-platform 3D engines

C++

http://gamedev.stackexchange.com/questions/21/easy-to-use-cross-platform-3d-engines-for-c-game-development

My Opinion (only for open source 3D engines):

• Irrlicht:
  • Light 3D engine
  • Clean C++ without dependencies and no STL.
  • Not very well documented but there are good tutorials.
  • Very small so you can customize it easily.
  • No OpenGL 3.X driver, Direct X 10.X or 11.X available in the official SDK.
  • Very good for mobile development;
  • Good community
  • Few extensions available
• Ogre3D:
  • Big 3D engine
  • It uses modern C++ such as STL, exceptions and RTTI
  • Good documentation (There are published books).
  • Many renderers (OpenGL, DirectX and OpenGL ES...).
  • Ogre3D have many extensions as CEGUI or Bullet integration, Tree nodes...
  • But more difficult to extend Ogre3D if you want something specifics.
  • Big community
  • Many extensions
• Crystal space: An old design and difficult to use. After few days, I stopped using this engine.
• Horde3D
  • Small engine
  • Modern design
  • Light community
  • No extensions
• Blendelf
  • Small engine with some dependencies
  • Modern design with moderns effects as DOF or HDR ...
  • OpenGL only
  • Light community
  • Bullet integration for physics
  • You use lua to pilot this engine3D

TL;DR:

• For a desktop game (or future commercial game): Ogre3D
• For a first game: Irrlicht
• For mobile development: irrlicht (Ogre3D is too big)
• For sexy effects: Blendelf

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.