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 Collegiate Dictionary

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