Regarding international medical volunteer opportunities, if you're in school or near a school, I'd ask the reference librarian; I've seen some great books on everything from names/contact info of international groups to books written by former volunteers on their experiences at my school's main library.
Or if it works out better to volunteer stateside, I had luck contacting local hospitals (wanted to gain substantial volunteer hrs when I have a few low-pressure months. Ended up volunteering in two hospitals around 24 hrs/week for several months, which worked well for my schedule. ps. my ER volunteer experience was the best in that it offered substantial patient interaction; in my OR role I was primarily supporting the medical staff, which was OK but patient interaction for the post-operation patients was minimal compared to the ER.
