Medical
Blue Shield of California Searchable database on health topics.

WebMD.com Information on health concerns and treatments.

Grey's anatomy brought to you by Bartleby.com

Mental Health
Personality Disorders from Mentalhealth.com Lots of other info and links.

Personality Disorders from Virtual Psychology online
Very characterizing.

Miscellaneous
The Writer's Medical and Forensics Lab
by D.P. Lyle, MD.  Contains articles, links to crime solving, forensic and medical and scientific experts, and a Q and A section.  Especially useful for writers of murder mysteries. 

Streetdrugs.org
Info about various recreational drugs.

Books
"HarperCollins Illustrated Medical Dictionary, 4th Ed." by Ida Dox.

"Howdunit Forensics" by D. P. Lyle, MD. A guide to forensics for writers.