Qt

Qt is a cross-platform, C++ GUI toolkit created by Trolltech, a Nokia company.

The largest project that relies on this toolkit is the KDE desktop, but Qt is also used in commercial software like Skype, Adobe Photoshop Album or MainConcept MainActor. It is also the foundation for Borland's Visual Component Library in both their C++ Builder and Delphi IDEs.

For software developers, Qt provides a high-quality, object-oriented API and a couple of tools to create user interfaces, translate applications and browse its comprehensive documentation.