The QDesign Music Codec was originally LBpack, by Dmitry Shmunk. QDesign corporation bought the codec and hired the developer to create what would become the official audio compression in Apple's QuickTime v3.
The codec's official name is "RaveSound", according to this press release. It's commonly called QDesign Codec. It is one of the most agressive codecs known - meaning it compresses quite well at very low bitrates, but it's aggressiveness introduces too many artifacts at higher bitrates.
The personal version comes with QuickTime Pro and allows encoding at bitrates up to 48kbps. The professional version allows bitrates up to 128kbps.
Apple is now steering away from proprietary codecs like Sorenson Video and QDesign, and focusing on standards (MPEG-4). So, it's unlikely that QDMC will last long.
QDMC's site at QDesign is available here.
You can obtain QDesign encoders at ReallyRareWares