OpenStreetMap creates and provides free geographic data such as street maps to anyone who wants them. 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.
Some interesting features on the site: you can perform search (addresses, cities, etc…), export an area in some formats (XML data, images).
You can use these maps for GPS navigation: for exampleCloudMade is an useful site with huge and updated maps database from worldwide in various formats (read the FAQ).