Notebooks Companion is designed to work with the brilliant Notebooks for iPad and Notebooks for iPhone by Alfons Schmid. Its purpose is to let you easily view and edit your notes that you have synchronized to your Mac using Dropbox. If you don't use Notebooks for iPad/iPhone and you don't use Dropbox, then it's probably not for you.
Note: As of December 23, 2011, Notebooks Companion is no longer available. I have received notice that Notebooks for Mac 1.0, the official app is now in beta testing. See Alfons Schmid's site for more information. I promised Alfons I would do this, and I did. Thank you for your interest!