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)

Memrise - Learn a new language with games, humorous chatbots and over 30,000 native speaker videos.

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

RemNote - All-in-One Tool For Thinking & Learning