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
Yahoo Dictionaries

English Dictionaries

One Look Dictionary Search: I use this when I need a quick definition or some basic synonyms for an English word.
Your Dictionary: Specialty Dictionaries: Find technical terms.

Japanese Dictionaries

Dictionary of Ibaraki Dialect
Dictionary of Japanese Surnames

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.

See also..

How to Choose a Good Dictionary: Checklist for making the right choice.