Tk is a pretty solid option nowadays, and it's wonderfully cross platform. Thanks to TTk, begone are the olden days when Tk apps stuck out like sore thumbs (the look and feel ain't an exact match even today, but nowadays the differences are more subtle). It also works with a remarkable number of languages. The only reason why I've reached for Qt instead lately is because the last couple projects I've done required an embedded browser window (i.e. because they're wrapping some web-based application) and Tk lacks modern options on that front (though I'd love to fix that at some point).

GTK+ and wxWidgets are other viable and popular choices for cross-platform desktop GUIs.