SpanishDict.com is a widely-used free Spanish to English dictionary and translator. The website has been around since 1999 and has millions of visitors per month who rely on their fast and accurate translation engine.
In addition to offering a dictionary and translation, the SpanishDict offer loads of useful tools for Spanish learners including a free Spanish course, multimedia flashcards, Q&A, Word of the Day, Spanish grammar guide, links to Spanish newspapers and radio stations worldwide, a phrasebook, conjugation tables, a chat interface and more.
The interface is clean and loads quickly. All in all, this is an excellent all-around resource for Spanish students, and is useful for teachers as well.