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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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Pros
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.