This is a site about kanji, created by Charles Kelly, who has made lots of online Japanese learning materials. Features include kanji reference and vocabulary lists, a kanji search function and kanji flash quizzes.
- In the 'Kanji Reference & Vocabulary Lists' there are external links to 10 other kanji study sites offering information on them, plus you can see vocabulary examples using your chosen kanji and try some quizzes.
- There is a Google Custom Search of just the kanji pages on the site, where you can look up kanji by entering the kanji itself, hiragana, katakana or English.
- There is an abundance of kanji quizzes.
About the 'Kanji Reference & Vocabulary Lists'
Kanji are collected into groups of 500.
Click on a kanji and you'll go to a detailed page where the meaning and some vocabulary words which use it are introduced.
As for the order in which kanji are displayed, once on a kanji page you can navigate by JLPT, newspaper frequency, or standard Japanese school order.

About 'Flash Quizzes'
You can try quizzes on 1,460 commonly-used Japanese words which are sorted by kanji.
These contents are one part of the Kelly brothers' Interesting Things for ESL Students.
One section of the Kelly brothers' Interesting Things for ESL Students' is 'Charles Kelly's Online Japanese Language Study Materials'(Introduction article). This can be accessed from the Japanese version of the site (Kelly brothers' Interesting Things for ESL ) only.
Here, aside from 'Kanji Study' you can find lots of other study contents for romaji, newspaper vocabulary, flashcards, the JLPT, videos and a massive collection of links. There are also plenty of quizzes on various topics!