Nonprofits are relatively underserved by tech workers. I've tried to get many of my friends to join me but the allure of bigco paychecks is simply too much. This is a pretty general answer but so's your question :)

I think if you pick a nonprofit with a mission that you feel passionate enough about to really give it your all then there's a good chance for you to quickly become a double-digit percentage of global technical productivity in a particular charitable domain.

For me it's been developing tools (mobile app and website) to help people with substance abuse issues communicate with their loved ones, themselves, and medical professionals.