The music review website, allmusic.com allows users to search for selections via emotion. They have hundreds of emotions listed, from angst-ridden to yearning. This is all thanks to "tagging", where music reviewers on the site are asked to enter the emotions that the music conveys.
It would be interesting to see this technology applied to shopping sites like Amazon.com, allowing users to find entertainment to suit their mood. It would also make sense for a site that allows instant downloads, iTunes for example could let users find music to suit their mood at that moment.
Tagging is appearing on user created community sites like Flickr. Here you can search photos by various tags. The presence of tags on Flickr allows for other applications to be over-laid on top of the site. One such application is Mappr, which allows for geographic based photo searching.
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