A Ruby GUI toolkit.
The web browser as a graphical user interface for Ruby applications.
While it's real, it's not in heavy development and is still alpha as of June 2006.
Tk is a GUI toolkit associated with the Tcl programming language, but is used as a toolkit elsewhere.
I primarily used it with Ruby, which it comes with.
FXRuby is a Ruby extension module that provides an interface to the FOX GUI library.
FOX (Free Objects for X) is a C++ based class library for building Graphical User Interfaces.