OS X 10.8 or later. This applet will assist with swapping, copying, and appending data between two user-chosen tags in selected tracks or tracks in the selected playlist:
Swap - swap data between tags, ex: ARTIST<->COMPOSER
Copy - copy data from one to another tag, ex: ARTIST->
Append - append data from one tag to the end of another, ex: ARTIST->COMPOSER - ARTIST
Prepend - prepend data from one tag to the beginning of another, ex: ARTIST->ARTIST - COMPOSER
The latter three actions also provide an option to delete the info from the first tag after the copy. Read Me explains all.
This update is free for registered users of v4.x.
Select the Action to perform between the two chosen tags in each selected track.
Locates pairs of track entries pointing to the same file
Append user-entered text to beginning or ending of selected tag of each selected track
Exports and then re-imports selected tracks' artwork
Batch merge and/or delete playlists
Export album artwork as folder.jpg to Album folder
Gather audio tracks whose files contain no artwork metadata