Ask HN: Languages/frameworks for desktop development: recommendations?

I've been doing some research for an internal enterprise application project and I'm considering the risks of adopting a non-web-based approach.

What do you think would be good choices of languages/frameworks to develop a "traditional" GUI desktop application? An "old-school" program containing a top-level menu and multiple non-modal screens, containing widgets like buttons, text fields and data grids? Primarily Windows based, but preferably multi-platform?

So far, I'm considering c#/mono/GtkSharp (not sure which fork), but I'd like to get more ideas.

It's been 8 years since my last desktop application project. It was c#/.net 3.5/windows forms.