Qt - Powerful, flexible and easy to use, Qt will help you not only meet your tight deadline, but also reduce the maintainable code by an astonishing percentage.
FLTK - Fast Light Toolkit - Fast Light Toolkit (FLTK)
JUCE - JUCE is a wide-ranging C++ class library for building rich cross-platform applications and plugins...
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
NoesisGUI - User Interface library for games and real-time applications.
wxWidgets - wxWidgets: Cross-Platform GUI Library