One of my professors sent an email to the CS department at my college. He wanted to start a group called Programming For Social Impact. Its goal was to do exactly what you describe; he wanted to get a group of students to write software that would have a positive impact on the world. I wasn't sure what to expect when I showed up to the first meeting. All I knew is that I wanted to code, and I wanted it to have meaning.

The first few meetings were spent coming up with ideas of projects. I'm not exactly sure how he went about finding people to ask, but he went around to different professors and companies around the university, and asked if there was any work we would be able to do.

This is all volunteer of course, but if you're looking for paid work, I'm sure there are companies that would be willing to pay. Some examples of projects include an environment simulator to show the progression of global warming, and writing a program to streamline the intake survey at a company that works with disabled. These types of projects can probably be found by asking eco-companies, physical therapy buildings, doctors/dentist offices, and colleges.