Britannica Online

Dictionaries
Links to a number of reference tools available online, including Roget's Thesaurus, Bartlett's Quotations, Strunk's Elements of Style, Webster's Dictionary, The Free On-line Dictionary of Computing and The New Hacker's Dictionary.  

Med Terms Medical Dictionary

MedLine Plus: Medical Dictionary

mediLexicon
Medical dictionary, ICD-9 codes, pharmaceutical companies, medical abbreviations, hospitals and associations

Merriam Webster's Online

One Look Dictionaries
This site checks several general-purpose and many specialized dictionaries to give you all the definitions you can handle.

Oxford English Dictionary Online

Oxford Reference Online: Premium Collection
Quick reference coverage of the full subject spectrum with the rich resources of the Oxford Companions.  It also includes quick and easy retrieval of short and long subject reference entries, bilingual dictionaries, English dictionaries, and quatations and proverbs.

Thinkmap Visual Thesaurus