The OpenStreetMap project, which has the highly commendable, if ambitious, aim of creating an open-source map of the world (as traditional streetmaps are under strict copyright) has just added a new viewer interface to its website. This replaces the flaky version it had before, and can be dragged around and zoomed into in a manner more like Google/Yahoo maps.
A quick click around shows that London and the Isle of Wight are the most filled-in areas, with a few other odd cities of the UK also having some detail. Presumably, these represent the areas where the current contributors to the site live. It's some way off being a replacement for any commercially-produced maps, but shows promise. Now they just need to make the editing interface simpler.