University of Utah
University of Utah: Hyper Hearts: Tutorials
This sites gives a great break down of the cycles of the heart. An animated cross section shows how the blood being pumped into each chamber causes sound and in turn affects an electrocardiogram.
Other
Dr. Saul: Biology in Motion
This interactive site provides animations and activities in several areas of physiology. The images are simple, but clear. The accompanying text provides a good description of the process being shown.