What is an Object Oriented desktop application framework similar to Rails?
My rule of thumb for personal (which I think this is) projects is to either use the language you're most familiar with OR the one you're most interested in learning.  Java, python and ruby are all great languages and MVC frameworks such as RoR exist for all of them.
That being said if you're interested in trying C# (which is very similar to Java) I'd recommend trying that with WPF for your GUI as it's both nice to use and very powerful.
I'm building a Rails web app but I would like to start building a desktop version but I don't know what language or framework to use. 
I've been thinking on developing it with Ruby too or Python or Java. I'm open to new languages as long as the framework is awesome and enjoyable to work with.
What LANGUAGE and FRAMEWORK would you recommend?