Thinking of creating a desktop application nowadays is really a pain. Multiple platforms and no definitive way to share the UI although no major functionality changes have been introduced in a long time.
Some options, for .NET framework, are listed at GUI Toolkits page at Mono Project web site. One interesting framework is GTK#, which uses the very-popular GTK UI framework.
While .NET has been introduced onto multiple platforms, it will be interesting to see if GTK# works equally well for a multi-platform UI.