My Opinion (only open source 3D engine):

 - [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++ 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]: old design and difficult to use. after few days, i quit 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

in few words:

 - for desktop game (or future commercial game): Ogre3D
 - for 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/