UPDATED: Save Album Art as folder.jpg v3.0
Save Album Art as folder.jpg v3.0 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.
Version 5.0 adds support for OS X 10.10 Yosemite and iTunes 12 (dropping support for pre-10.8), adds "Check for Update" menu command, and has some minor maintenance tweaks.
More information and download is here.