Good article:

For five years, people have been contributing to OpenStreetMap – the Wikipedia-like attempt to create a free, user-generated map of the world.

Thanks to the project, millions of people now use free maps online or on their phones, and OpenStreetMap covers large swathes of major cities around the globe. In fact, it’s got so much information that some users in Germany are now plotting incredible amounts of detail – such as the location of individual trees.