xforms -- XForms library
http://world.std.com/~xforms/
XForms is a GUI toolkit based on Xlib for X Window Systems. It features a
rich set of objects, such as buttons, sliders, and menus etc. integrated
into an easy and efficient object/event callback execution model that
allows fast and easy construction of X-applications. In addition, the
library is extensible and new objects can easily be created and added
to the library.