The latest Development Build of Mp3tag features a web sources framework which allows tagging via almost any website which offers album information. Please note, that this part is still in beta stage.
To import tags from a website, a web source description file is needed, which contains general information about the source as well as parser scripts which are responsible for parsing the right data out of the WWW sites.
I've written the web source description file Discogs.src as an example how to use this framework and hopefully some of you will pick up the idea and write a source description file for e.g. your favorite online music catalog or a Lyrics site.
Please have a look at this brand-new help topic for documentation of the web source description file format and please don't hesitate to ask, if you have any questions!
Hope you'll like it and use it