The illusion of life
Cento Lodigiani demonstrates the 12 basic principles of animation developed by the 'old men' of Walt Disney Studios, amongst them Frank Thomas and Ollie Johnston, during the 1930s. Of course they weren't old men at the time, but young men who were at the forefront of exciting discoveries that were contributing to the development of a new art form. These principles came as a result of reflection about their practice and through Disney's desire to use animation to express character and personality.
This is also a great primer for user interface designers.