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.