wxRuby is a GUI toolkit for Ruby. It uses native GUI widgets on Windows, OS X and Linux/GTK, giving users the look, feel and behaviour they expect of desktop applications. At the same time, developers only need to write code once to work on all those platforms.
It's easy to install and bundle, and is offered under a liberal licence that's compatible with free and commercial development. It's based on the latest stable version (2.8) of the mature and fully-featured wxWidgets C++ library.