Brainscape is an excellent free iOS app for studying any topic based on mobile flashcards. This "smart" flashcards get rated by the user and based on those ratings will differentiate instruction to help the student learn. Best of all, a user can either browse through and purchase decks of flashcards or create their own and even share their cards w/ others. This is great for collaborating, study groups, and for educators looking to create classroom flashcards for their students.
I highly recommend checking out Brainscape by clicking here!!!
For a more detailed review check out Graphite.
For more info check out Edshelf.