UPDATED: Save Album Art as folder.jpg v2.0
Save Album Art as folder.jpg v2.0 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" file here to use as display artwork.
This latest version is written as a Cocoa-AppleScript Applet and can only run on OS X 10.6 and 10.7 systems (the previous universal version is available from the script's entry page). It consolidates all settings into a single preferences pane.