Open Street Map
From Just Solve the File Format Problem
Open Street Map was created as an open, noncommercial alternative to proprietary commercial map services such as Google Maps. Like Wikipedia, it is open to public participation to add or change map data in areas you know about, and in some cases it has managed to accumulate more complete and accurate data than Google has in particular places. The data is released under a free license, so that it may be used in all sorts of applications by end users and software developers without fees or permissions.
Various file formats are used including XML and JSON; the data is made freely available in a number of forms. The website has browseable, zoomable maps just like the commercial competitors.