openstreetmap

This isn't just a Drupal thing, but a great Open Source development for everyone using mapping (and there are moves to use it in Drupal - http://drupal.org/project/mapstraction).

OpenStreetMap.org is an exciting example of a open source geographic resource. It's a kind of wikipedia for maps - a free editable map of the world, and anyone can add roads, bike paths and other geographic data to it. Around the world, many are contributing to it. In some places, mapping parties are being held to spur the collection of data.