An EJ/JE dictionary claiming 750K words. Classifies them by topic.

Among other things, weblio seems to have a good stock of pharmaceutical names.

Online access to 43 Japanese-language dictionaries. Appears to have a lot of current and specialist vocabulary.

Slick web interface to the ubiquitous EDICT project.

A wide range of vocabulary and personal names, collected and edited by hundreds of people. The dictionary files can be downloaded for offline use.

Find a term, then look down the left pane. Often you will find a corresponding page in a foreign language. Be careful as the "corresponding" pages do not always correspond exactly. Also check out the sister project Wiktionary for dictionary entries. The translations are often hidden, so you have to "show" them.

A 10億訳例プロジェクト with probably the largest JE/EJ database available. Don't forget to select the second row of technical dictionaries if needed!

The same old Eijiro, but in a new beta engine that adds more terms and the ability to do inclusive and exclusive searches: dog cat looks for entries with both words, and dog -cat excludes "cat" from your results.

Eijiro is a popular dictionary that can accept both English and Japanese words or phrases. The output generally includes translations for the word itself and translated sentence pairs of the word in context.

Tom Gally's volunteer J-E dictionary project for difficult-to-define words. Thanks to Adam Rice for this link.