You'll all be aware of my enthusiasm for maps in general. Here's another interesting resource - well, interesting to me, anyway - an interactive map of Britain of a similar type to Google Maps or Multimap, but using old Ordnance Survey maps from the 1940s (which are now out of copyright and hence in the public domain). So we're pre-motorways and pre-Beeching, and therefore the landscape looks quite different.
It doesn't quite have the rich functionality of the other two sites (mouse-dragging of maps and mouse-wheel zooming are handy features, for instance) but it's fascinating nonetheless. You can also contribute to the project by finding your place of residence and providing your postcode, thereby enabling future visitors to search for locations by postcode. If you happen to know mine you should now be able to go straight to it. Here's central Newport to get you started.
While we're on the subject of maps, if you do want current OS maps (and who doesn't?) then I recommend Dash4It, who not only seem to have a constant 30% off offer on the go, but also offer free postage. Bargain.