The wildly popular application MacJournal by author Dan Schimpf includes a hidden preferences panel that is activated simply by applying the following command via Terminal:
defaults write com.DanSchimpf.MacJournal IncludeHiddenPreferencesMenuItem YES
After applying the aforementioned Terminal command, start MacJournal and look for the menu item MacJournal -> Hidden Preferences. You'll now have a whole slew of hidden settings available for your perusal. Documentation of the various settings is available here on Dan's site.
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