Rikaichan is a popup Japanese-to-English/German/French/Russian dictionary add-on for Firefox and Thunderbird.
Denshi Jisho is an easy-to-use and powerful online Japanese dictionary. It lets you find words, kanji and example sentences by searching in many ways.
This is a free online dictionary for learning the kanji of Japanese used in daily life. You can look up the details and pronunciations of the kanji.
Teaches the stroke order of the kanji using animated images and written descriptions. The kanji entries also include common compounds and 四時熟語.
A monolingual Japanese slang dictionary.