OpenStreetMap

 OpenStreetMap  is "a free, editable map of the whole world that is being built by volunteers largely from scratch and released with an open-content license". The dataset that is used to build the map is free to download and reuse, and users can contribute data back to OpenStreetMap requiring nothing more than a user account.

History
OpenStreetMap was founded in 2004 by British programmer Steve Coast who collected the first GPS trace by cycling around Regents Park in London. Mapping parties started springing up with people collecting GPS traces for the road map by driving, cycling, walking and rollerblading around cities, and collecting notes and photos of features that ought to go on the map.

Data quality
OpenStreetMap quality varies based on country. The United States tends to lag some way behind European countries, possibly as a lot of OpenStreetMap contribution comes from cycling enthusiasts. The UK, Netherlands and Germany are particularly well-developed.

Government data has often been used in OpenStreetMap. In the US, the TIGER dataset has been used, as well as state and city datasets.

APIs etc

 * Nominatim
 * OpenMapTiles, a self-hosting solution for rendering OSM maps on a website
 * Photon - self-hostable point of interest search engine based on OSM with an API, including reverse geocoding in its API

"Products"

 * Wheelmap, which displays wheelchair accessibility data and contributes collected data back to OpenStreetMap
 * Humanitarian OpenStreetMap Team, which co-ordinates OpenStreetMap contribution for humanitarian purposes, providing up-to-date maps to help with assistance efforts in countries facing natural or medical disasters including the West African ebola outbreaks and the 2010 Haiti earthquakes
 * OpenFireMap, which shows a map with fire hydrants entered into the OpenStreetMap Database

IndieWeb overlap

 * User:Tommorris.org uses OpenStreetMap-based maps for the 'places' section on his site, and is also looking into ways to use OpenStreetMap data to power checkin.