Heinle's Newbury House Dictionary of American English contains over 40,000 entries.
With simple, clear definitions, this online tool provides a wealth of sample sentences and idioms.
The Longman Dictionary of Contemporary English (LDOCE)
The Merriam Webster Dictionary and Thesaurus is an online dictionary and thesaurus. In addition to definitions, synonyms, homynyms, etc., there are many fun learning tools that include
A user-friendly way to learn new English words and expressions.