MP3Pro
From Hydrogenaudio Knowledgebase
A combination of MP3 and SBR created by Coding Technologies. It delivers quite good quality at bitrates lower than 96kbps, and is backwards compatible with players that only support plain old MP3 (but the SBR part won't play on these players, so quality won't be nearly as good)
It's a proprietary and closed format, with no documentation or source code available.
