There are organizations like "Doctors without borders" and Federal Emergency management agency out there. But typically they look for experience. For physicians, 2 years of experience AS AN ATTENDING is a requirement I believe. I'm sure you could volunteer in some capacity but as a med student (or even resident), it probably would not be as a medical professional. Look into Christian missions trips...World Vision is one example. There are many out there.

Sharptool is all over it. If you want to volunteer to do ortho specifically, you are going to be out of luck. Most of those programs are looking for experienced surgeons to teach the local surgeons, although there are some opportunities for senior level residents. Usually, they are not looking for surgeons to operate.

If you are willing to volunteer to help out in general (not providing any significant level of medical aid), there are tons of opportunities available, but you'll have to pay your own way and most likely support yourself while you're there.

Check out Orthopaedics overseas at:
http://www.hvousa.org/pcntry.cfm?ProCode=OO