I imagine that when you're considering a career move, it can be helpful to hear the perspective of the people looking to hire you, so I'll venture an answering as the Executive Director of a nonprofit. We use Tech as tool #1 to serve our mission and are always looking for devs, ux designers, etc. There's no natural place I would "obviously" go to find devs who are interested in social causes. The talent market lives mostly in word of mouth, although we do post periodically in craigslist or on the job sites like linkedin and monster. In general, we do go to hackathons, socially-oriented accelerators and incubators (such as CivicX, code for america, or even the hub), and we post job listings at the universities. It's not a very robust set of cause-specific work. At the same time, we're in a race for talent just like everyone else, so when we're looking for people who can truly accelerate our impact, we try to poach talent. The pitch goes something like this: "You're obviously doing awesome stuff at company X. Why don't you put your considerable talent to better use and help us change the world for real?"

If I can make a suggestion, if you can get a sense for what cause(s) you care about most (education? health delivery? poverty alleviation? something else?) and start to explore the organizations serving those causes, you'll certainly find your way to a job posting here or there for an organization that truly excites you. And excitement is what makes a good match when you're doing work that makes the world a better place. Good luck.

(edited for grammar)