Nihongojouzu

Japanese Language Proficiency Test (English Site)

Japanese Language Proficiency Test (Japanese Site with practice exam questions)

Kanji a day

Kanji site

Hiragana Megane (allows you to view the furigana in Japanese websites)

Yookoso - Kanji and Grammar for JLPT

MLC Meguro - Free online Japanese study material

Manythings.org - A comprehensive list of Japanese study sites