Dictionaries - General

The American Heritage® Dictionary of the English Language (Fourth Edition)

Merriam-Webster Dictionary

Merriam-Webster Word Central (for children)

OneLook Dictionary Search (search 847 dictionaries at once)

Dictionaries - Specialized

Free Online Dictionary of Computing

Merriam-Webster Medical Dictionary

Patterweb: Early Modern English Dictionaries Database Search Utility

Thesauri

Merriam-Webster Thesaurus

Roget's Interactive Thesaurus