Wolfram Research
Wolfram Mathworld: Astroid
Find out how an astroid is related to the ellipse at this math page. This page starts with describing how an astroid may be created easily with two concentric circles with a given ratio, the site then goes into details regarding the...
Other
Gobierno De Navarra: Cicloides Y Trocoides
This unit shows the graph of a polar equation and gives several pretty graphed examples. It shows the difference in the graphs just by varying a constant value.
Other
Old Dominion U.: Trochoids, Hypocycloids, & Epicycloids
Has discriptions of Cycloids, Trochoids, Hypocycloids, and Epicycloids are made.
Cut the Knot
Cut the Knot!: Interactive Mathematics: Geometry Articles, Theorems, Problems
Contains over five hundred Geometry articles, problems, puzzles, and games. Some sites relating to circles are provided within this list.
University of St. Andrews (UK)
University of St. Andrews: Hypocycloid
Shows the graph and the parametric cartesian equation. The site also includes links to graphs of related curves and definitions of the associated curves.