Qt is a great cross-platform development toolkit. Very powerful and much easier to use than C++. Bindings available for many scripting languages. For a sizable GUI app written in Qt on Windows, "porting" to Linux and Mac OS X only takes changing very few lines to get the different compilers on the different platforms to do the same work - less than an hour and you're running on all three OS-es with full desktop integration (menubar/taskbar applet icons working correctly - in the menubar on OS X and near the clock on Win/Lin).
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Linux, BSD, Android, iOS, Windows, Mac, Blackberry ... Qt has 'em all.
Pro Good documentation
Professional, thorough documentation with examples, available either in a web browser or in the stand-alone desktop client called assistant.
Pro Global community
The community behind Qt is both massive and approachable. Digia (also owners) are joined by the likes of Intel, KDAB, ICS, Canonical and numerous others in sponsoring development, while communities such as KDE also contribute significantly. Forums are active, mailing lists are open, irc channels chatting, git repositories well managed. Answers to questions are usually minutes away.
QML is "HTML5 done right": a declarative language that integrates beautifully with C++ code when necessary and exposes the power of the GPU ... simply the best way to create a modern GUI in terms of effort and results.
Pro LGPL license
Pro Open Source
Qt is licensed under a OSS license through a dual-license policy.
Con Not free/expensive
Free version is limited and prohibitive.