This toolkit used to be called "wxWindows" -- it was renamed wxWidgets after MicrosoftCorporation "approached" them.
Its API design is similar to that of MFC but much more consistent. 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 many other programming languages such as Python, Perl and Ruby. It relies upon a system's "native" ToolKit, so it gives you the LookAndFeel of that platform. For example, on MicrosoftWindows it uses the MicrosoftWindows look, on Unix it can use GTK, Motif? or raw X11 (for wxEmbedded), and so on for the Mac and hand-held wx ports. (You can probably even compile and link against the MicrosoftWindows port of GTK!)
wxPython is a very fast way of prototyping and developing graphical applications, if you are familar with Python. It is fairly easy to get the hang of the required coding style. The latest stable version (at the of writing) of wxWidgets (which also includes the GTK, MicrosoftWindows, and Embedded ports) is 2.4.0, released in January 2003.