Many of these "secret" menu items first began as items concocted by restaurant employees or customers. Others are discontinued former menu items that are still available for those in the know.
Restaurants are increasingly selling off-menu items and using them as marketing tools that make customers who feel like insiders want to share the information with their friends.
But in a contradictory move, many of these companies are showcasing these secret menus on their own websites. Mama Fu's Black Market menu is one example of this. (The company's Thai Fire Curry Dish is shown here.)
The trend is being driven by restaurants, such as Panera and BurgerFi, that want to speed up the process for filling complicated orders and to cater to dietary preferences.
Click ahead to see some of the most interesting secret menu items we could find.
—By CNBC's Katie Little
Updated 26 March 2014
Posted 8 April 2013