If you are interested in getting your hands dirty around the world, you might checkout Engineers without Borders[1] or the Peace Corps[2], or one of many similar non-profit organizations that employ skilled people to help others around the world. You might even volunteer with the Red Cross to help victims of the typhoon in the Philippines.
On the less philanthropic end of things, there are a host of organizations solving problems in the biomedical world. From hospitals to biotech companies, there are many possibilities. I've found working in this space incredibly fulfilling, especially given that I've had a chance to see patient cases where we can make a difference.