The name should give you a hint on this one (a person with an eidetic memory is able to recall images, sounds, or objects with amazing detail). This app uses a technique called spaced repetition to get users to remember things like words, quotes, and facts. Spaced repetition may not sounds familiar but it's likely you've used it. When people are trying to learn something new, they often quiz themselves on it a few times in relatively quick succession to make sure they're remembering it correctly. Once they've got it down, they'll wait longer and longer between these quizzes, a form of spaced repetition. [free; iOS]