File this one under the 'never would have thought of that' category! MaildropGuide.com is a giant web directory that indexes all of the mail drop boxes within the United States, as well as a number of other countries as well.
A mail drop is a secure address that is recognized by the postal service, where business owners that need re-mailing services can rent a box and use it as a legitimate postal address. As well, any business owners that are currently operating a mail drop can be listed in the directory for only $40 for a lifetime listing. I think this will be a very profitable directory, they've chosen a unique and very targeted niche. I even noticed a listing in Honduras!