Wolfram Research
Wolfram Math World: Cardioid
The site opens with a visual representation of how a Cardoid, a one-cusped epicycloid, is made. The site then shows the parametric, Cartesian, and polar equation for the special epicycloid. The last half of the page gives more equations...
University of St. Andrews (UK)
University of St. Andrews: Cardioid
This site, which is provided for by the University of St. Andrews, gives a brief history of the cardioid. Shows parametric and cartesian equations.