1. Merriam-Webster OnLine
A free, searchable on-line dictionary and thesaurus, word games, a word of the day, and many other English language and vocabulary reference tools and resources.

2. Cambridge International Dictionaries
Free online dictionary from Cambridge University Press.
dictionary.cambridge.org

3. WordReference.com
Dictionary containing English to/from Spanish, French, German, and Italian, and a language forum.
www.wordreference.com

4. Dictionary.com
Free online dictionary search, translator, word of the day, crossword puzzles and word games, and vocabulary learning resources for many languages.
dictionary.reference.com

5. LEO English-German Dictionary
Allows two-way searches for over 400,000 words and phrases. Several options available.
dict.leo.org