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The LGPL version of Qt is a cross-platform application framework for C++. It's also includes QML, a CSS & JavaScript like language.
It is very popular and seems to be leading the pack today.
SpecsUpdate
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.
Pro Best documentation of any development platform
The comprehensiveness of the Qt5 documentation is setting a new bar in the industry. Not only does it explore the entire toolkit in incredible depth, but it does so without being obtuse or using an excess of unfamiliar language.
Pro PRO IDE QtCreator
Qt offers a free IDE for development named QtCreator. It offers all required tools for progessional software development not only in C++. It supports natively CMake based projects (one of the most used build system) and can import complete build environment from the build cache. It can be used with non-Qt-GUI / non-GUI projects with the same full language (and framework) support as with Qt-based projects.
Cons
Con Non-standard C++ features
Signal/slots and introspection are managed by a C++ precompiler, the meta object compiler (MOC).