Standard GUI Framework for C is a framework for building GUI applications in ANSI/ISO standard C in a platform independent (i.e. portable) manner. The framework currently only targets X11, but there are plans to target both Microsoft Windows (Win32) and Apple Mac OS X (Carbon) after the initial release is done.
Information regarding Project Releases and Project Resources. Note that the information here is a quote from Freecode.com page, and the downloads themselves may not be hosted on OSDN.