It's long been a frustration of mine that the maps and mapping data collected by the British Ordnance Survey are tightly controlled under Crown Copyright and not available for novel use by anyone who doesn't have lots of money to trump up. There are lots of interesting but unprofitable things that can be done with maps: we've seen a lot of examples using Google's map embedding API, but with access only to the map imagery and not the underlying data it's hard to do anything more than just throw points on the map.
OpenStreetMap is attempting to change all this by hosting a grass-roots effort to collate mapping data under a creative commons share-alike licence. Their coverage is still patchy, but for major areas like central London and Birmingham the maps are looking quite complete. The map data is collected using a combination of track logs from GPS devices, out-of-copyright maps from the 1940's, and more recently the aerial photos hosted by Yahoo!'s maps service. Consequently the data is probably not as accurate as the Ordnance Survey data, but good enough for many purposes.
On a related note, the imaginitively-named Old Maps is an online interface to scanned maps from the 1880s — or there abouts, depending on what area you're browsing. Apparently these maps from over 100 years ago are still somehow under copyright and most reproduction is prohibited. This is particularly frustrating as they seem to have scanned all of the maps in black and white rather than greyscale, making most of the captions very difficult to read. It's sad that these historical documents are still being hoarded by the Ordnance Survey after 127 years, since an OpenStreetMap-style grass roots effort is — barring the invention of time-travel, at least — impossible for these.