Published in 1954, Fitts’s Law is one of the most important governing rules of good interaction and usability design. It’s a pretty complex and multi-layered concept, and while easy enough to explain in words, to really get a good understanding of it, one really needs pictures. The stunningly designed Particletree has a very good visual breakdown of the law, and some good commentary on best practices for its application. If you do any sort of UI design, you really do need to check this out.