You can search for site by subject or level
[Reading] Site search （25）
On this site you can try quizzes on Japanese grammar, vocabulary, kanji, listening and reading comprehension and JLPT (Japanese Language Proficiency Test) questions. As well as English, the site has support for Chinese and Korean.
Here you can read copyright-free works of literature (and more) written in Japanese. These include some famous masterpieces.
Here you can study a range of different aspects of Japanese - grammar, vocabulary, conversation, reading, listening and so on - from beginner to intermediate level.
Lots of resources and information for students and teachers of Japanese. Includes plenty of practice tasks and tests.
An audio-visual resource site designed by Tokyo Metropolitan University. There is an abundance of YouTube content, useful as well for those who study alone.