There's a really fun website called "The Covers Project" here:

http://www.coversproject.com/about

that is attempting to find the longest "chains" of covers; ie.

Artist A covered Blah, Blah, Blah by Artist B
Artist B covered La La La by Artist C
Artist C covered Sis Boom Bah by Artist D
etc, etc, etc

They have a good definition of what a "cover" is, and a link to the Wikipedia page.