American Association of Public Health Physicians
http://www.aaphp.org/

Association of Clinicians for the Underserved
http://www.clinicians.org/

Doctors Without Borders
http://www.doctorswithoutborders.org/

Los Médicos Voladores
http://www.flyingdocs.org/

Physicians Committee for Responsible Medicine
http://www.pcrm.org/

Physicians for Human Rights
http://physiciansforhumanrights.org/

Physicians for a National Health Program
http://www.pnhp.org/

Physicians for Peace
http://www.physiciansforpeace.com/

Physicians for Social Responsibility
http://www.psr.org/

World Medical Association
http://www.wma.net/