ArtLex
Art Lex: Rhythm
This site from Artlex provides a thorough definition of the Principle of Design known as Rhythm. There are links to other sources and terms as well as visuals from artists such as Klee, Matisse and Mondrian that show further examples.
Minneapolis Institute of Arts
Arts Connect Ed Artist's Toolkit Encyclopedia: Rhythm
An interactive site that explores the elements and principles of visual art through the artist's toolkit. This encyclopedia entry on rhythm and movement covers rhythm and repetition, alternating rhythm, and rhythm and movement.