This form of tagging has the rather clumsy name of a folksonomy.
A Folksonomy is a system of classification derived from the practice and method of collaboratively creating and managing tags to annotate and categorize content; this practice is also known as collaborative tagging, social classification, social indexing, and social tagging. Source: WikipediaThese folksonomies give us a picture of the collective view, a summary of the zeitgeist, an understanding of the popular interpretation of meaning. There is personal meaning, and shared meaning. By looking at a tag such as happiness on flickr you can explore the collective meaning of the word and associated tags clustered around it.
Equally, by exploring the tag happiness on Technorati, you get an equally interesting array of associated tags, a list of the bloggers most frequently using that tag, and an understanding of the how the word is interpreted or used.
This bottom-up type of tagging is likely to be real way that the semantic web starts. Sir Tim Berners-Lee W3C is trying valiantly to get a top-down system of tagging to happen but that is currently unlikely to happen.
It's going to be a tricky affair making all of this work. Both of the images below came up from a image search on Google using the keyword "freedom"
Semantics and semiotics are complicated sciences and I am far from claiming any kind of breakthrough, but I am clear that understanding how to interpret the collective intelligence will be important. These tags are the same tags that are keywords in search engines results that pull from website metadata, blog post labels, and file names, and other sources. Coming up with clever ways of interpreting this data is going to be pretty important going forward.