Dictionaries and Thesauri 

Merriam-Webster Online

Advertisement-supported content from one of the most popular publishers of reference works. Provides full entries from Merriam-Webster's abridged dictionary, thesaurus, and Spanish/English dictionary.

American Heritage© Dictionary

One of the many full-text works offered for free (but advertising-supported) viewing by Bartleby. Entries here are full-length, taken directly from the current edition of the respected American Heritage© Dictionary.

Oxford English Dictionary Online

Searchable full text of the current edition of the most respected dictionary of the English language, especially notable for its detailed etymologies. Please note that access is available only from computers on campus or logged in to a Missouri S&T VPN.

Roget's II: The New Thesaurus

Another free (but ad-supported) full-text work offered by Bartleby. Entries here are full-length, taken directly from the current edition of one of the most popular thesauri.

Synonym.com

Ad-supported site offering fast, clean access to synonyms and antonyms of many common words. Not as authoritative or information-rich as other resources, but handy for quick searches.