This site has an interactive visual toolkit that explores:
* Visual elements and principles: line, color, balance, space, shape, and movement/rhythm
Each element/principle has three sections:
* Watch an animated demonstration
* Find examples of the concept in works of art from museums
*Create your own composition
A fun site to explore, there is also a section where you can see artists in action and an encyclopedia which "is an in-depth guide to learning more about the building blocks of composition. Here you'll see many examples of works of art that illustrate the visual elements and principles."
Via Beth Dichter