I am considering creating a GUI-based tool that I want to be cross-platform. I've dismissed Java, as I personally do not like Swing. I'm currently considering C# and using Mono to make it cross-platform. However I'm wondering whether new-fangled cross-platform languages like Ruby can offer me a decent GUI development environment.
Have you looked at SWT on Java? It uses native widgets and is much easier to get a nice interface with it than Swing.
I strongly back Qt for cross platform GUI development. It's awesome and APIs are very intuitive. GUI look and feel almost native with Qt because it uses GUI controls provided by underlying OS.
Though the basic interface with C++ other language bindings are available. For Ruby RubyQt is available. Unfortunately it's very immature.