You're premed so your life plans may change (blah blah blah) but who the hell am I to argue with Bob Saget?
Any "core" specialty is going be gangbusters in the third world, obviously: family/internal medicine/peds/ob-gyn, you've got opportunities out the whazzoo. Subspecialty surgeons also have their niche: From my limited experience, they tend to do more work in urban centers there that I've seen, but they still see people who are really desperate who are sent there.
Radiology is going to be a bit difficult, but I heard of a mammo program for some disadvantaged areas and I've heard of some rads goes doing philanthropy stuff: trying to get GE and Siemens to donate CT scanners and MRI's overseas. But any field like pathology, radiology, etc. having completed medical school you still know more about medicine than 99% of the world's population, so if you're really jazzed to go overseas you can play internist and treat people pretty decently, and your lifestyle and financial rewards may make that more palatable than a family doc who may not have the cash to go and do as much charitabl ework as he would like.