1. Unuseable CD produced during failed attempt at writing to writeable or re-writeable CD media. Certainly related to the coaster-like shape of a CD, and the relative value of these failures. “I made a lot of coasters before I got a good CD.”
2. Useless CDs received in the mail from the likes of AOL, MSN, CI$, Prodigy, ad nauseam.
In the U.K., beermat is often used in these senses.