OpenSource 3D Endgine

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

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

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.

Small engine
Modern design
Light community
No extensions

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

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