Anki - Anki is a program which makes remembering things easy. Because it's a lot more efficient than traditional study methods, you can either greatly decrease your time spent studying, or greatly increase the amount you learn.

Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.

Simply Learn Languages - A phrasebook of 50+ languages for traveling (with audio)

LearnObit - A tool for your learning, like Anki & Workflowy in one

Kommit - Flashcards / Spaced Repetition System with text-to-speech

Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.