There's QT, which is huge and unwieldily and not so good looking on OS X, GTK+ which is a mess on both Windows and OS X, wxWidgets (based on other libs), and a few fringe, and incomplete libs.
For one-platform apps, though, or a single backend for multiple platform UI frontends, it will do just fine, provided we get bindings for Cocoa, Windows UI libs, GTK etc.
Crystal at the very least has good support for binding C so maybe this will be the language which finally produces a modern gui toolkit, but with the move towards web apps its becoming increasingly unlikely.
Well, the whole Cocoa/QT/WinForms scope might be huge, but we don't even have good minimal UI libs -- e.g. with just the few basic widgets: buttons, text fields, labels, radio buttons, checkboxes, kind of what HTML forms can do -- and only very limited functionality (e.g. no fancy text formatting in the labels or whatever).
Tons of programs could be GUI-fied with just the above (sort of what Tk does, but decent looking).