Virtual Mentor is the American Medical Association's free online ethics journal. It explores the ethical issues and challenges that students, residents, and other physicians are likely to confront in their training and daily practice.  

The theme of the December 2006 edition was Global Health: Ethics of International Medical Volunteerism and it has a lot of good articles and clinical cases for students to read and reflect on before they begin their first international assignment. 

Check it out: 

http://virtualmentor.ama-assn.org/2006/12/toc-0612.html