WP MediaTagger is a WordPress plugin by phd extends the concept of post based taxonomy natively supported by WordPress toward an media based taxonomy.
WP MediaTagger is an extensively configurable plugin providing all the tools required to categorize your WordPress blog medias by associating it to any defined tag or category. Among many features, it comes packed with a tagcloud widget that will make the process straightforward for everyone and will permit a transparent integration. Initially developped to tag medias under the WP ImageTagger denomination, WP MediaTagger now covers a much wider scope and will tag as well any media supported by WordPress.
Plugin homepage: http://www.photos-dauphine.com/wp-mediatagger-plugin