XForms is a graphical user interface toolkit for X 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.

WWW: http://xforms-toolkit.org/
