WaniKani - Learn around 1,700 kanji and 5,000 vocabulary words in about a one to two years.
LearnObit - A tool for your learning, like Anki & Workflowy in one
Kanji Koohii - Remember the kanji; mnemonics, smart dictionary, flashcards, spaced repetition.
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.
Kanshudo - Master kanji, hiragana and katakana, along with Japanese grammar and vocabulary.
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.