It's a web browser layered on top of your game.
libRocket uses the time-tested open standards XHTML1.0 and CSS2.0 (while borrowing features from HTML5 and CSS3), and extends them with features suited towards real-time applications.
Can be integrated in any engine (or application) since it relies on you to provide the backend, including rendering, texture loading, etc.
There used to be an official website as well as proper documentation.
CEGUI has been around since 2003.
The current version of CEGUI is licensed under the MIT License.
The AntTweakBar library is free to use and redistribute. It is released under the zlib/libpng license.
GiGi is available under the LGPL license, which is available here.