I have been writing software, pro-bono, for non-profits for over a decade now. I get most of my "work" through idealist.org, which is a trove of opportunities. You might have to monitor for a few months before you find just the right project for you. Be patient.

My most recent gig, however, was found through the United Nations volunteer program. I can't find the link atm but I'm sure it'll come up for you with a bit of Googling.

It has been incredibly rewarding, and you might be able to write off some of your computer, conference, books, internet and travel expenses.

I love this kind of work for many reasons, but from the developer's perspective it's rewarding because the client is always very grateful for what you are doing and there's never a deadline, so there's always time to do it right and not take shortcuts.

I have traveled to some very interesting places, albeit mostly (but not always) at my own expense, with this work. I am fortunate to be in a financial position that I can do it, and I can't recommend it highly enough.