Global Changes to MP3 Libraries

[Cadaverous Pallor]

iTunes, like mousepod said. What you're describing is easily handled with iTunes and doesn't require fancy tricks. If you're annoyed at the World genre, just sort by genre so World groups together, then select large chunks of music (ctrl-click) and hit ctrl-I to see the Multiple Item Information (Get Info) screen. You can make up any genre you want. I've done this to my "Electronica/Dance" genre because that's not specific enough for me.