"...The project was started because most maps you think of as free actually have legal or technical restrictions on their use, holding back people from using them in creative, productive, or unexpected ways." OSM Wiki.
OpenStreetMap, often shortened as OSM, is a collaborative project to create a free editable map of the world. Anyone can create an account and start editing the data anywhere in the world. The data can also be exported and used in your own mapping applications.
The Map: http://www.openstreetmap.org
About OpenStreetMap: http://wiki.openstreetmap.org
Open Street Map Foundation: http://osmfoundation.org