CEGUI is a powerful GUI library but I am not sure (since you did not clarify) what you mean by lightweight. It contains a lot of headers, too, but they are all related to the GUI or the rendering of it. On my computer, the current Release built in default VS2010 Release mode takes about 3 MB in total for the dlls. There are of course assets that take space too, but clearly these may vary depending on your needs. It uses XML and image files in the assets, the images take the most space. It requires a bunch of dlls depending on your requirements (png, jpeg, xml parsing dlls) but they are all quite small in Release. So unless you are irrationally pedantic about having too many DLLs in use, this should be no issue.
It is written in C++ (0.7-1.0 will be based on C99 standard, 2.0+ will be in C++11 standard) and supports dynamic and static linking. Dynamic linking is the supported type on Windows. Static linking on Windows is not recommended for CEGUI.
Here a short summary of features:
Modern (uses STL, XML for its asset files, and internally supports C99 standard for maximum compatibility, after the 1.0 Release it will be using C++11 in the development branch). It also has support for unicode characters and all sort of languages (as can be seen in the samples).
Offers the following widgets out-of-the-box: text fields (scrollable), editboxes, multiline editboxes, spinners, checkboxes, buttons, radio-buttons, tabbed windows, progress bars, scrollable panes, sliders, dropdown-menus (comboboxes), menu bars, layout containers (vertical/horizontal) and more
Published under the Open Source MIT license, one of the licenses offering you the most freedom, you will only need to distribute the license file with the application and that's it
Layouts can easily be created and edited using a WYSIWYG editor written in Python. It is called CEED and is also freely available. It has gone through a lot of testing and is suitable to be used for production. A stable Release has been published and new features are in development for the future. An imageset editor is also available in the same application.
As stated, this is free for commercial use. The only obligation you have is to distribute the license file.
CEGUI is fit for cross-platform use. It officially supports Windows, Linux, Mac OS. It has also been used on iOS and Android. Android is supported in the development branch.
It was originally designed as a GUI library for games but it is also perfect for usage in scientific applications, rendering applications, visualisation. Desktop applications can be done as well. A simple Windows-like skin is available in the default assets.
It has been around for over 10 years, has an active community and is a robust, feature-rich and extensible library.
