Multiple tags on music in iTunes?

Is there any plugin for iTunes making it possible to tag the music with multiple tags (not ID3-tags or genre)? Just like last.fm does, e.g. "soft electronic african female-vocalist"? Genres are so totally unusable to use as a music categorizer.

I use iTunes a lot for syncing various iDevices and streaming music to the Airport Express, so I'm not really interested in an alternative app, even though I might be if it supports more or less everything iTunes does and have some more features, but also being compatible with and updating the iTunes library.

A big bonus would be if it's also allowing hierarchical tag structures.

While it won't satisfy the hierarchical feature you'd like, the Grouping (or even the Comment) field in iTunes could be used to hold tags. Ideally you would want to do something to make the tag always unique by separating by a comma or say <> around the word. You can then use a Smart Playlist to filter for this tag. (Grouping contains).

There is an unfinished program called QuickTag that leverages that ability and makes it easier to tag.

There is also a hint on MacOSXHints.com that shows how you can grab Last.FM's tags and apply them to the current playing track. You'll need to modify their script slightly - namely to use the grouping or comment field (and match anything special you've done, eg. <>s around the word) instead of the genre. The alternative Perl version worked faster than the original script for me.