Idealist.org is an interactive site where people and organizations can exchange resources and ideas, locate opportunities and supporters, and take steps toward building a world where all people can lead free and dignified lives. They have introduced me to Net Squared, an organisation with a mission is to spur responsible adoption of social web tools by social benefit organisations.

Net Squared have a mashup comp on right now with cash prizes. All you have to do is to combine two existing online technologies to create something more socially useful. Link >
Examples given include Everyblock which filters an assortment of news and information by location so people can keep track of things happening in their neighbourhood.