I Hate to say it, but Windows Media Player has a nice feature in its media library. Adding an entry in a field using \';\' as a seperater, ex:
Louis Armstrong; Ella Fitzgerald Will show the same track in playlists for each artist, but eliminates the situation where you have an artist named Louis Armstrong and Ella Fitzgerald, wich just doesn\'t look right. This works for most of the columns in the id3 table. genera, album...