del.icio.us now has gotten a bit smarter about media types. URLs ending in particular extensions automatically get tagged with a few special labels. For example, .mp3 will automatically be labeled system:filetype:mp3 and system:media:audio. Apparently you can also use these special labels yourself on any old URL.
Bonus podcasting boosting effect. Any RSS feeds that include one of these magic tags become enclosure containing. I'd have to work through the math, but I think with some creative tagging this can be an instant podcast creation tool, presuming of course all the media is already on the Web.