Sage lets you build rich, highly functional, cross platform web-enabled desktop applications and applets by simply marking up the UI and attaching JavaScript (or Ruby, Python, etc.) event handlers. You simply point sage to a URL and it downloads the markup and accompanying scripts and renders the application or applet in real-time (the same way a browser renders documents). All that is required to run Sage is a Java Virtual Machine (v1.5 or later, v1.6 preferred).
Monkeybars is a library enabling you to write GUI applications using JRuby and Swing. You can build your GUI in any editor so that you can have an application that is designed using modern tools but has all the logic contained within Ruby.