The online dictionary service from Google supports definitions, audio pronunciation guides and translation into multiple languages. A bilingual dictionary can be very useful because it gives you translations for the many possible meanings a word might have.
There are two dictionaries on my system that contain almost identical words & spelling corrections but are stored in different locations and maintained separately.
Visuwords is an online dictionary and thesaurus that helps you find word meanings and explore the association between different words.
Live Dictionary for Instant English definitions and meanings.