Album Art Downloader XUI is a program designed to make life easier for anyone who wishes to find and update their album art for their music collection. It can search multiple online sources for album art. New sources can be user-defined by script files. It can be fully controlled through command line parameters, to allow for integration with applications such as Foobar2000 or tag management software. It can also analyse files to find Artists and Albums from tags or filenames, and check if album art is already present. If you use Foobar2000, then it can read the media library or playlists from that application instead.
Thanks to Xinok for the tip!