I've put together a plugin which downloads metadata from libraryThing.com into calibre, based on the amazon one.
The plugin is using the webapi from libraryThing so the information that is extracts can be sparse, however the field I have found usefull is the series and series index fields.
It searches with the isbn if available otherwise uses the name of the book.
the fields it processes (if available) are:
In order to use it you will need to get a developer key for librarything, see http://www.librarything.com/services/keys.php
Hopefully someone will find it usefull,
Any comments/tips appreciated as this is pretty much the first chunk of python I've ever hacked together.