Japan Knowledge (Duke only)
SmartKanji.net
Denshi jisho
Goo Dictionaries
Kanji Networks
Pop Jisyo
Reading Tutor
Rikai
Yahoo! Japan Dictionaries
Rikaichan Firefox plugin that translates Japanese text on mouseover.
Learn Kanji
Applications for the iphone/ipod touch
Even with the multitude of electronic dictionaries, print dictionaries are still useful. See this guide for Bilingual Dictionaries, 国語, 漢和辞, 現代用語 and 古語 as well as
擬音語擬態語 Japanese language -- Onomatopoeic words -- Dictionaries
ことわざ Proverbs, Japanese -- Dictionaries -- Japanese
外来語 Japanese language -- Foreign words and phrases -- Dictionaries
類語 Japanese language -- Synonyms and antonyms -- Dictionaries
方言 Japanese language -- Dialects -- Dictionaries