Today, in conjunction with Adaptive Blue, Deri (NUI Galway), Faviki, Yahoo!, Zemanta and Zigtag we launched Common Tag, a new format for tagging content on the web.

Using tags to organize content has become a well understood practice for content creators, site owners and users alike. While keyword tags provide a useful way for individuals to categorize content according to their personal interests, keyword tags don’t provide a strong sense of meaning and thus are of limited use when aggregating content across heterogeneous collections of information. To solve this problem Common Tags provide a uniform way to unambiguously express the meaning of a tag.

Common Tag takes advantage of RDFa, a W3C recommendation for expressing structured data in HTML using strong semantics. The Common Tag format builds on this standard, allowing a content developer to state what concepts are described within a document, an image, a video or even a fragment of a paragraph. Anything that can be identified with a URI can be tagged.

The real power of Common Tag lies in its ability to precisely state the meaning of a tag. This is done by pointing to web resources which acts as a strong identifier for a concept. Since Freebase provides strong identifiers for every Topic it contains, you can use any of the nearly 6 million Topics in Freebase to specify the meaning of a tag.

As an example, if I wanted to add a Common Tag to this blog post, I could say that this article is about the Freebase Topic /en/commontag using markup like:

<div xmlns:ctag="http://commontag.org/ns#" rel="ctag:tagged">      <span typeof="ctag:Tag" rel="ctag:means"         resource="http://rdf.freebase.com/ns/en.commontag"/></div>

And because Common Tag is build upon RDFa, you can extend the model to suite your needs by mixing in other RDF vocabularies such as FOAF to describe who created the tag, or SKOS to describe how the tag relates to other tags.

Common Tag metadata is the core around which other services and applications can build. For instance, search engines are making Common Tag content more discoverable. Yahoo! is already indexing Common Tag metadata as a part of its Search Monkey and BOSS offerings. Sindice, a semantic search engine, is also indexing Common Tag markup providing applications access to Common Tag data through its API.

Faviki and Zigtag, services that let users bookmark pages with strong identifiers, are making use of Common Tag’s design that lets anyone (not just the content creator) tag any resource on the web with a precise statement about the contents of that resource. Both of these systems are adding Common Tag output to each user’s bookmarks.

And Zemanta is extending its publishing tools to help content managers effortlessly apply Common Tag metadata to their publications.

While the companies involved in the launch of Common Tag each play an important role in this new tag ecosystem, we expect that the real power of Common Tags will be revealed as the community of content creators, application developers and UX designers start building systems around Common Tag metadata.

To learn more about Common Tag visit the web site commontag.org and join the community discussion in the Common Tag Yahoo! group.