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

 - [Irrlicht][1]: 
 - 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][2]: 
 - 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][3]: An old design and difficult to use. After few days, I stopped using this engine.
 - [Horde3D][4]
 - Small engine
 - Modern design
 - Light community
 - No extensions
 - [Blendelf][5]
 - 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



 [1]: http://irrlicht.sourceforge.net/
 [2]: http://www.ogre3d.org/
 [3]: http://www.crystalspace3d.org
 [4]: http://www.horde3d.org
 [5]: http://blendelf.com/