Features

CEGUI

Mature codebase, CEGUI has been around since 2003!
Multi-platform support - Windows, Linux and Mac OS projects/solutions are available
Support for both 32bit and 64bit architectures (MSVC, GCC, clang, XCode, MinGW-w64)
Heavily data-driven by using XML files, a generic property system and powerful skinning abilities
High flexibility, you can code your own widget renderers, use your chosen XML parser, script module, etc...
UNICODE support - utf8 and utf32 (check the Fonts sample)
Many different renderers supported. We currently offer implementations for OpenGL 2.X, OpenGL 3.2+ Core Profile, Direct3D 9, Direct3D 10, Direct3D 11, Irrlicht, Ogre3D and OpenGL ES (experimental). A dummy renderer (NullRenderer) is provided as well. Additionally there is support for Crystal Space (provided by the Crystal Space team) and Open Scene Graph (provided by the Open Scene Graph team).
Clear interface in case you want to write your own renderer
Several tools, including a Layout- and Imageset editors
Many wiki pages and useful sample projects and application templates
Commercial products have used CEGUI already. Check the Projects Using CEGUI.

The CEGUI unified editor, includes project management and multi-file/multi-tab editing.