Build Cross-Platform Ruby and HTML Desktop Apps with Bowline
Bowline respects MVC, you can design your views in HTML5/CSS3 - then bind them to your Ruby models. There's no request/response cycle - any changes in models automatically get reflected in the view.
Alex has clearly put a ton of effort into this. Despite being cross-platform, though, don't get too excited if you plan to target Windows.
An interesting by-product of Bowline is rbyte, a Ruby 1.9.(1|2) library that "compiles" Ruby 1.9 source down to byte code and makes it easy to reload and use said code.