Use Qt4. http://qt-project.org/
This is a self containing framework which contains developers tools, GUI builders, String/IO/XML/Thread classes, Audio/Video controls, HTML widgets and many, many more features. It's built to be completely multi-platform, one code for all systems.
In contrary to wxWidgets, it feels more object-oriented, and has by far better documentation and its better maintained.
See it online: http://qt-project.org/doc/qt-4.8/
EDIT-
6 years since the original Answer. I should point to Qt5 - http://www.qt.io/