The FOX toolkit (Free Objects for X) is a cross-platform C++ toolkit with language bindings for Ruby, Python, and Eiffel. It has a clean, simple API based on message passing, and does not rely on native widget sets.
The list of platforms is growing! Currently, we have FOX running on a large number of operating systems, ranging from Linux, FreeBSD?, SGI IRIX, HP-UX, IBM AIX, SUN Solaris, DEC/Compaq Tru64 UNIX, to MS-Window operating systems like Windows 9x, Windows NT, Windows ME and Windows 2000. Since most of the FOX implementation is completely oblivious to the underlying platform (in many cases it is not even including header files), applications work virtually identically on all these platforms.Fox seems to be gaining some ground. It is under fairly active development. The Ruby binding is terrific. See FoxMdiSample for an MDI application written in FXRuby.