Today I created a "Quality Check" plugin for the request from schuster a few posts above. It depends on some code in the next release of Calibre so I will release it this weekend. As per the screenshots it allows you to search for books in your library that have covers below certain thresholds in file size or dimensions, title/author sorts that need updating or ISBNs that are invalid. If anyone can think of other checks they would like to see on this let me know.
In the toolbar screenshot you can also see the icon for the "Manage Series" plugin I mentioned earlier on this thread. I sent a beta version to a few testers last night, anyone else interested in providing feedback on it just PM your email address and I will send you a zip. That will probably be released later this week too.