Recs.
Updated
Qt is a cross-platform GUI library, it is free for open source projects but very expensive for commercial use (starting at $459/month).
Specs
Pros
Pro Extremely mature
With origins at Trolltech in 1991 and then with Nokia when they were at their zenith (remember when almost everyone had a Nokia cell phone?), plus adoption by some of the heaviest hitters in software development not named Microsoft/Apple/Google (think KDE, Wolfram Mathematica, etc.), it has evolved into a powerhouse among GUI implementation frameworks out of a need to remain useful to those demanding and innovative institutional customers.
Pro best programming toolkit ever: the power of C++ with the elegance of Java; plus QML
Cons
Con It feels unfinished
Shouldn't have a paid license; QML has very strong foundations, very easy to extend and customize but the default controls (especially for desktop) look awful. Digia didn't have the strength to finish it: the default Tabs control has a really unintuitive API and looks very raw (the tabs don't even have the close (x) buttons. Missing controls for treeview or gridview...So much wasted potential, it's a shame...