From Shockley Consulting:
TuneTags is an application that works with iTunes to let you add keywords, or "tunetags" to your music.
Have you ever realized that many songs do not fit into just ONE Genre? Or, you want to note which songs are good for Driving, are Romantic, or just about anything else useful to know? You can make playlists for these things, but it would be better if the song itself could be marked with many different keywords, or "tunetags" that let you know more about each song. Then, smart playlists could be made that show you all your good 80's Driving music, or Romantic Folk music, or Defiant Rock music or even music good for getting psyched before a competition.
TuneTags lets you mark each song using many tags from a list, or Library, of tags. It comes with a starting Library of tags, but you can add your your own to the Library as you think of new tunetags that describe your music.
See Developer Note below for an explanation of how TuneTags saves the tags to a song.