mtMintKit is a GUI toolkit that I created from scratch so I could more easily create SDL programs.
It is written in C language so it is portable across many platforms. I have created generic runtime environments for GNU/Linux and Windows systems which are able to run the various example programs I have created.
mtMintKit allows GUI programs to be written for SDL much more quickly than if they were to be written from scratch. However, unlike other toolkits it is designed to be small and minimal in order to keep applications modular and make maintenance much simpler. This minimal approach also embraces the concept of customization through the creation of widgets and objects either inside applications or shared via external C libraries.
As well as basic GUI widgets, mtMintKit also offers utility functions for handling fonts, UTF-8 strings, balanced binary trees, timers and inifiles. Click HERE for the HTML documentation.
I have deliberately kept the library modular so I can easily use parts of it in other projects. For example I have used parts of mtMintKit in my spreadsheet API/GUI mtCellEdit without any SDL dependencies (I am using GTK+ as the GUI with the mtMintKit modules for font rendering, trees, inifiles, and UTF-8 handling).
mtMintKit is free software, which means you are able to freely use, modify and distribute it according to the GNU General Public License.