Flickr has a new extension to their tagging mechanism: machine tags. As far as I can see, it's a minor extension to the tagging syntax, allowing for a colon separated prefix and then keyword/value pair separated by an =, e.g bmd:some=value. All the real action though is in the fact that Flickr's search API actually leverages the machine tags for more sophisticated queries. Seems like a slow drift towards the semantic web.