Agency for Healthcare Research and Quality (AHRQ)
The lead Federal agency charged with improving the quality, safety, efficiency, and effectiveness of health care for all Americans.
http://www.ahrq.gov/

MedHelp.org
Community of health forums providing medical advice on a wide range of health topics. Available in Spanish.
http://www.medhelp.org/

Medic8
A leading UK medical information website for healthcare professionals and consumers around the world.
http://www.medic8.com/