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.

WaniKani - Learn around 1,700 kanji and 5,000 vocabulary words in about a one to two years.

Kanji Book - Learn Japanese online with kanji mnemonics. Study all of the jōyō kanji with the best spaced repetition system. It's easier and better than any flashcard app.

Kanji Koohii - Remember the kanji; mnemonics, smart dictionary, flashcards, spaced repetition.

Repetitions - Repetitions is a cross-platform spaced repetition system (SRS), an intelligent flashcard application that makes learning and memorization easy.

Kanshudo - Master kanji, hiragana and katakana, along with Japanese grammar and vocabulary.