You may not have heard of Super Audio CDs (SACD) before, which is understandable since it was a high definition format that was never able to really take flight due to the release of the DVD. SACDs require different players than a regular CD player, but the technology has been integrated into a few DVD players.
I recently tested my first SACD and was rather impressed with the quality and the much richer experience of a multichannel mix on my 5.1 surround sound home theater. It has a much wider and expansive feel than a regular stereo CD mix and I'm sure it was more fun for the mixers to put together. It is almost a shame that the SACD never grew in the marketplace, but we do have DVD-Audio discs that are much more common with a similar higher quality compared to CDs. My next step will actually be testing one of these out. It may be interesting to see if there is any (perceivable) difference between a DVD-Audio disc and an SACD.
Even though the format never made it mainstream, SACDs continue to be made, primarily for classical or instrumental albums. There is an excellent site for browsing and searching albums available in SACD at www.sa-cd.net.