Quizlet
A flashcard and study tool app that makes studying languages, history, vocabulary and science simple and effective.  Listen to your material pronounced correctly in 18 languages. Teachers, help keep your students engaged and motivated.
Quizlet Overview  https://www.quizlet.com 
Quizlet tutorial  https://www.youtube.com/watch?v=Jo6ITr9yt4Y 
 
SmartCards +
Make your own flashcards using text, images and sound or pre-made decks from popular apps like Quizlet.  Keep tabs on your progress with interactive statistics and performance reviews.  Smart keyboard switching for language learners.
SmartCards+ Overview  http://www.smartcardsplus.com/ 

Chegg Study
Homework help app with expert answers.  Snap a picture and submit it to get details answer in 30-45 minutes.  Search their library with over 26 million fully solved homework questions.  They will show you a step-by-step solution.  Learn with guided video explanations.  Create and study your own digital study decks or search over 500 million flashcards already in the library.

Brainscape
Find, create and share Smart Flashcards. Brainscape partners with top publishers, schools and educators to create flashcards for subjects as diverse as foreign languages, MCAT, Music Theory, SAT prep, and AP Exams.  You can create multimedia flashcards on their website and within the app.  Syncs across Brainscape’s website and all your iOS devices.
Brainscape Overview  https://www.brainscape.com/

MindNode
Mind mapping app for Mac and iOS.  It helps you capture your thoughts as words, images, links, tasks or notes.  Move, tag, connect, detach, and reconnect as often as you like.  Focus Mode, Visual Tags and foldable branches allow you to concentrate on one thought at a time.  Style your mind map with personalized themes and stickers. Share and export your documents.
MindNode Overview  https://mindnode.com/ 

iThoughts
Mind mapping app also available in Mac and Windows versions.  Enables you to visually organize your thoughts, ideas and information.  Includes task lists, summarizing, brainstorming, project planning, goal setting, and course notes/revision.  Import and export from many popular desktop application formats.
iThoughts Overview  https://www.toketaware.com/ithoughts-ios 

MindMeister
Mind mapping app helps you organize, prioritize and generate new thoughts. Free or personal plans.  Syncs with free online account.  
MindMeister Overview  https://www.mindmeister.com/