UPDATED: Save Album Art as folder.jpg v2.2
Save Album Art as folder.jpg v2.2 will export the artwork of the selected tracks or tracks in the selected playlist as a JPEG image file named folder.jpg to the folder which contains each selected track's file--presumably the track's Album folder.
Many third-party music players and systems (Sonos, for example) may look for a "folder.jpg" here to use as display artwork.
This latest version is basically a maintenance update and is now a stay-open app that will float above iTunes while running.