The new england journal of medical http://www.nejm.org/.
Journal of the american medical association (JAMA) http://jama.jamanetwork.com/.
the LANCET http://www.thelancet.com/.
British medical Journal http://www.bmj.com/.
Scientific american http://www.scientificamerican.com/.
Scielo http://www.scielo.org/ y http://www.scielo.org.pe/