GTK+ is a cross-platform widget toolkit for creating graphical interfaces. The toolkit scales from the small, one task programs to large, complex applications, and provides the necessary framework for working in your favourite programming language.
This site aims to keep up-to-date documentation for language bindings, allowing anyone of any experience to utilise GTK+.
Contributions can be made at GitHub.
A PDF version of the tutorial is available for download.
The source can also be fetched from GitHub.