Here are some dictionaries that you may find useful.
Bilingual (Japanese-English) Dictionaries
ALC Bilingual Dictionary: This dictionary is good for looking up idiomatic expressions and checking usage.
Excite Japanese/English/Chinese Dictionary
Goo Dictionaries: These are good for checking the basic meaning of words and learning the readings of compounds.
Infoseek MultiDictionary: Dictionary that lets you search by for words in English or Japanese and kanji by stroke, reading, radical, or the character itself. Allows partial matches.
Internet Resources for Translators
Jeffrey’s Japanese-English Dictionary Server
PSP’s Bi-Directional English Japanese Dictionary
Sanseido Web Dictionary
Kanji (Japanese Characters) Dictionaries and Reading Assistance
Jim Breen’s WWWJDic: This is the best kanji dictionary for beginners. Offers many different ways to look up characters.
Kakasi: Kanji Kana Simple Inverter
Pop Jisyo: Web-based pop-up dictionary for Japanese, Chinese, Korean, and other languages.
How to Choose a Good Dictionary: Checklist for making the right choice.