For some of the more commonly used GUI toolkits VGUI provides an easy way to create menus, dialogue boxes and add tableaux. Using this VGUI pattern to create your applications also means that they are easily ported from one GUI toolkit to another. Thus it is possible to compile the same application to run with a different GUI on different platforms.
GUI toolkits that are supported or are intended to be supported in the near future are:
Information at http://www.gtk.org/
Information at http://www.troll.no/developer/
Information at http://www.opengl.org/Documentation/GLUT.html