Jim Breen’s WWWJDIC
Jim Breen's WWWJDIC is the most comprehensive English ↔ Japanese dictionary online. It is tremendously useful for looking up words, finding kanji, and anything else that could be necessary. You can search Japanese words with romaji, kana, or kanji, and it recognizes many phrases and idioms and also provides examples. The main page can be accessed in both English and Japanese.