Well, the title says it all, but I need to spam someone so I can make my first related post editable.
What the heck, since the subject caught your eye...
LibraryThing Match (LTMatch) is a calibre plug-in for comparing your calibre e-book list with your LibraryThing™ library. It can be used to track which e-books exist in both, and easily identify books missing from either.
Cheers.