You can also participate in hackathons organized by Random Hacks of Kindness.
From their website:
Random Hacks of Kindness (RHoK) is all about using technology to make the world a better place by building a community of innovation. RHoK brings software engineers together with disaster risk management experts to identify critical global challenges, and develop software to respond to them. A RHoK Hackathon event brings together the best and the brightest hackers from around the world, who volunteer their time to solve real-world problems.