Skillied - Learn locally
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.
ACT Flashcards from Magoosh - Everything you should have learned in high school
Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.
FlashTabs - Flashcards in your new tab screen
Memrise - Learn a new language with games, humorous chatbots and over 30,000 native speaker videos.