GTK+, or the GIMP Toolkit, is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK+ is suitable for projects ranging from small one-off tools to complete application suites.
Everywhere! GTK+ is cross-platform and boasts an easy to use API, speeding up your development time. Take a look at the screenshots to see a number of platforms GTK+ will run.
GTK+ is written in C but has been designed from the ground up to support a wide range of languages, not only C/C++. Using GTK+ from languages such as Perl and Python (especially in combination with the Glade GUI builder) provides an effective method of rapid application development.