MusicBrainz has a Folksonomy cloud that can be used to tag artists, releases and tracks with anything you like. Jaikoz uses this to add genres at releases and track level. Because genres are subjective this feature is going to become more useful as more people submit their genres, so please give it a go. Jaikoz also uses the folksonomy cloud to fix the genre field when correcting songs from MusicBrainz.