iTunes Multi-Genre Tagger adds descriptive metadata to your music. These descriptive tags are gathered from the Last.FM API (no user account necessary) and then written into the 'Grouping' field of your music file(s).
And wherefore? To break old habits. With the 'Grouping' column displayed in iTunes, this enables you to filter music by multiple categories, even very specific ones (i.e. "90s argentina indie") from within the iTunes search feature. In this way you'll find music you'd otherwise not think to listen to.
Here is what your 'Grouping' column will look like in iTunes afterwards:
This project is now maintained on Github. You may download the source there and even continue development if you like. Enjoy.
A previous version was released that works only on 10.4 and 10.5 can be found here: Download (version 0.6)