An audio CoDec (for use with the Ogg FileFormat) intended as a successor to MP3, offering better compression and better audio quality while being unencumbered by Patents (MP3 is). OggVorbis files support up to 256 channels, variable bit rate and all other important features.
In 
a test conducted by c't (the biggest German computer magazine; article is in German) OggVorbis has also been proven to offer decidedly better quality at very low bitrates than any of the competing audio CoDecs. At high bitrates, MP3 and OggVorbis seem to be tit-for-tat.
It is supported by the latest release of RealPlayer (version 10) and the associated suite of tools thanks to HelixCommunity.
10 pages link to OggVorbis:
