If you already know the Esperanto word, ReVo is an excellent dictionary, taking its definitions from the Plena Illustrita Vortaro. It is run as a wiki, so you’ll find translations in national languages that users have provided.

Taking from ReVo and other pages, and allowing you to search in Esperanto and English is ViVo. This is a Wiki, so if you spot a word missing you can add it yourself.

Lernu’s frontpage has a customisable dictionary. It’s on the right-hand side. Use the drop-down to choose the input and output languages that you desire.