OpenGUI:

Easily integrated with any input system: All input is injected into the system by the application using simple function calls.
Cross API: Is easily used in native DirectX or OpenGL, as well as the OGRE rendering engine.
Easily extendable widget set by class inheritance.
Auto scaling, pixel aligned fonts, rendered via FreeType 2.
Free form render operations allow Widget imagery to be rotated and scaled in any way imaginable.
Plugin system so custom widgets can be used in any application without recompiling.