Goodhack is a Hack day being held in London on the 9th May. The challenge is to write an application which makes the world a better place. Even if you can't come to the event, you can add your problems to the list to be solved (http://www.goodhack.org/problems/add)
That is a great thing and I totally thought of making the same website, because I'm constantly looking for a good problem to solve. I hope this gets popular, because there's a lot of problems out there hackers don't even know about waiting to be solved.