OneLook Dictionary Search Tool
You can search a word's meaning in over 112 dictionaries at the same time, from the Oxford and Cambridge to Turkish, Yiddish, German, Russian and Old English.
Medical Encyclopedia of the U.S. National Library of Medicine
Browse diseases by starting with the first letter of the illness, then select the disease.
Medical Dictionaries @ the National Library of Medicine
A nice cross-section of choices, medical and otherwise.
Latin Dictionary and Grammar Aid
This is a handy reference. Many english words start with latin roots like a-, de-, ab- etc, and knowing the latin definition makes it much easier to parse out the meaning of an unfamiliar word. I'm eternally greatful I took Latin in high school!
Merriam Webster Medical Dictionary
Merriam-Webster Dictionary & Thesaurus
Thesaurus.com
Yale's Writing and Style Guides
Yale's Dictionary LINKS
Yale's Encyclopedia LINKS