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 ;-)