Otk (Open Tool Kit) - portable library for graphical user interfaces (GUI's) based on C and OpenGL. Emphasizes simplicity without eliminating capability. Supports panel's, text label's, buttons, form boxes, pull-down menu's, slider control's, etc..
I need to create simple fast apps with a gui for Linux and sometimes for Windoze too. I looked at FLTK but didn't like the bloat that c++ added which, for my embedded apps would be a problem. The OTK percentage paradigm takes a bit of getting used to but does deliver the end result in a predictable way so I wouldn't change it now. It has saved me hours of work for sure.