Dictionaries and Thesauri

American Heritage Dictionary
Lists synonyms, usage notes, or examples of usage taken from published works of well-known writers

Merriam-Webster Online
Merriam-Webster Online includes a dictionary, a thesaurus, and audio files to help with pronunciation.

Oxford English Dictionary
Definitions, origins, and quotations showing each word’s range of meanings from the time it entered the language to the present

Modern Languages: Oxford Reference Online
Spanish, French, German, and Italian dictionaries plus much more

Medical Dictionary: Merriam-Webster
Definitions of terms related to medicine, nursing, psychology, and pharmacy

The Glossarist
A searchable directory of glossaries and topical dictionaries

Roget's Thesaurus II
Contains 35,000 synonyms and over 250,000 cross-references in an easy-to-use format