From Jakob Nielsen, usability guru. The user's experience should be similar to all other similar applications to minimize learning curve and maximize usability.
Mac OS X, which combines a powerful core foundation with a compelling user interface called Aqua. With advanced features and an aesthetically refined use of color, transparency, and animation.
lecture notes for the course Programming Graphical User Interfaces (PGUI). The course belongs to the curriculum of the Department of Computer Science in the University of Tampere.
a free library providing windowing and widgets for graphics APIs / engines where such functionality is not natively available, or severely lacking. The library is object orientated, written in C++, and targeted at games developers who should be spending t
a high-level multi-platform, thread-safe C/C++ windowing and graphics library built upon a fast, low-level graphics kernel. (new feature is FGX - dynamic GUI via XML file aka XUI file format)
C++ interface for the popular GUI library GTK+. Highlights include typesafe callbacks, widgets extensible via inheritance and a comprehensive set of widgets.