OpenStreetMap

OpenStreetMap has the tagline "The Free Wiki Worldmap" and allows you to view, edit and use geographical data in a collaborative way from anywhere on Earth.

Its wiki main page says the project started because most maps we 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.

Its main discussion place is its mailing lists but it also has a web based forum.