An example of a startup doing work for the developing world is Wave, focusing on helping people send remittances to family in west Africa.
http://www.angazadesign.com/watch-video/ (obligatory: http://careers.stackoverflow.com/company/angaza/)
and many, many others, e.g.
https://welldone.org/ http://saner.gy/ http://wecyclers.com/ http://www.allpowerlabs.com/ ...
not to mention areas of "SF tech proper" with a direct positive impact, such as medtech or cleantech.