wxWindows, found at [http://www.wxwindows.org], is a cross platform [GUI] ToolKit. (It also has its own classes for non-GUI stuff like networking, string handling, file manipulation, graphics processing, and printing, to name a few). It is mainly used as a library with C++, but has bindings to other programming languages such as Python, Perl and Ruby. It relies upon a systems 'native' windowing libraries so it gives you the [LookAndFeel] of that platform. For example, on Microsoft Windows it uses the native toolkit, on Unix it can use GTK, Motif or raw X11 (for wxEmbedded), and so on for the Mac and hand-held wx ports. 

  

 wxPython is a very fast way of prototyping and developing graphical applications, if you are familar with python. It is fairly easy ([IMO]) to get the hang of the required coding style. The latest stable version (at the of writing) of wxWindows (which also includes the GTK, MSW, and Embedded ports) is 2.4.0, released in January 2003.