Georgia State University
Georgia State University: Hyper Physics: Kepler's Laws
An outstanding page describing Kepler's three laws of planetary motion.
NASA
Nasa: Kepler's Third Law
This site from NASA states Kepler's third law of motion and extends it to develop an equation for the velocity of an orbiting planet.
Cornell University
Cornell University: Astronomy: Orbital Motion and Kepler's Laws
At this site from the Astronomy Department of Cornell University, Kepler's three laws of planetary motion are stated. There are brief explanations of each, along with links to additional information on related subjects.