Ask HN: As an engineer, how can I volunteer in the developing world?

I'm an experienced software developer from the states, but I've spent a lot of time overseas, particularly in Asia. I'm interested in spending some time, at least six months, volunteering in the "developing world", for lack of a better term. Are there programs or organizations that I should be aware of that will let me use my skills to help others? How can I have the biggest impact?