This has been discussed many times on this site. Do a search for international volunteer programs and check the international forum.
The American Medical Student Association has a list of <a href="http://www.amsa.org/global/ih/ihopps.cfm" target="_blank"> international health opportunties</a> although most are for current medical students.
Check <a href="http://www.volunteerabroad.com" target="_blank">volunteerabroad.com</a> for a list of international volunteer organizations -- many of them are health related.
If you are into a longer comittiment try the <a href="http://www.peacecorps.gov" target="_blank">Peace Corps</a>. They have many of their country programs are health-related.
Good luck!