The AAP's site leads you to the Section on Community Pediatrics and the Section on International Child Health. Both worthwhile.

Social Medicine - Great online journal

Association of Clinicians for the Underserved

Migrant Clinicians Network

World Health Organization Commission on Social Determinants of Health - Final Report. Closing the Gap in a Generation: health equity through action on the social determinants of health.

South Texas Environmental Education and Research (STEER) Program-Harlingen
Open to students from accredited U.S. medical and health professions schools with a focus on public health and environmental health issues.

Along Side a Border - Mike Seifert's blog  

Society for Equity in Child Health (SECH)