XForms (X11 graphical user interface toolkit)

XForms is a graphical user interface toolkit based on the X11 Xlib library. 
It's written in C and allows to easily write GUIs for programs. For that, 
it comes with a lot of widgets (buttons, menus, input fields, scrollbars, 
you name it) as well as fdesign, a tool that lets you create a GUI using a 
GUI. In addition, the library is extensible and new objects can easily be 
created and added to the library.

Note: the XForms toolkit hasn't anything to do with the W3C standardized 
XML XForms format which unfortunately also is called "XForms".  The XForms 
toolkit already existed long before the XML XForms format stuff was invented 
and those newcomers just picked the same name ;-)