(As usual, click for hi-res.)
The most comprehensive, topographic mapping covering all of England and Wales from the 1840s to the 1950s. Two editions for all areas, and then regular updates in the 20th century for urban or rapidly changing areas.
Fantastic resource, especially when you can overlay historic maps over modern ones, and tweak the transparency.
(Of course, there are plenty of excellent maps of Scotland there too. :-)