Free Map Repositories?

mapless asks: "I have recently begun working on a car-based computer for music and other tasks. I thought that it would be a good idea to include mapping software. The problem is that I do not want to be tied to specific commercial packages (which are mostly Windows based), and I don't want to be tied to a specific OS (this will start out as a Windows machine and migrate into a Linux system when I learn more). I thought about just downloading a LOAD of images from Yahoo or Mapquest, but that would be pointless and redundant. Is there a map database that one can freely access, possibly with sample code on how to manipulate the map? I think this is valid for Web sites, not just a car computer, as well, since all maps on the Net are from propietary databases."