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.