Common Tag is an open tagging format defined using RDFa that makes content more discoverable. Unlike free-text tags, Common Tags are references to unique, well-defined concepts, complete with metadata and their own URLs.
The companies involved in the development of Common Tag are AdaptiveBlue, DERI (NUI Galway), Faviki, Freebase, Yahoo!, Zemanta, and Zigtag.
More information on the Common Tag Website.
