I would suggest looking at visualruby. Its an IDE to create GUIs using ruby. It uses GTK as its graphics toolkit, and the apps look great to me on any platform. You can see screenshots on Win7, WinXP, Ubuntu, and Mac on the screenshots page:
http://visualruby.net/site/Screen%20Shots.html
You can easily try it on every platform by installing visualruby on various platforms, and running the example programs.
The checked answer was written before visualruby was released.