YourDictionary
YourDictionary is a dictionary you can understand with lots of tools to help you choose your words precisely, understand a word, avoid usage pitfalls and be confident in your use of words.
Cambridge Dictionaries online
Searches Cambridge Dictionary of American English, Cambridge International Dictionary of English, Cambridge International Dictionary of Phrasal Verbs or Cambridge International Dictionary of Idioms.
The Acronym Server
Roget's Thesaurus
Lexical FreeNet Connected Thesaurus
Includes unusual language resources such as a "Rhyme Coercion" thesaurus and a "Related Word" thesaurus.