Ushahidi / MapBox

Both Ushahidi and MapBox are open source mapping applications that allow users to upload custom tiles and data. When paired with their news-aggregating software (Swift River for Ushahidi, and Managing News for MapBox), they act as crisis-mapping applications which allow people on the ground to tweet or text a crisis type and location, which is then relayed to an organization that can help (like the UN). Crisis-mapping is a ground-breaking social innovation, and has saved many lives. Think of it as a "vote-up" international 911.

The Swift River stack is Ushahidi, Kohana PHP Framework, and FrontlineSMS. The Managing News stack is Drupal, MapBox, and SlingshotSMS. For other differences, you'll have to do your own research.