This might have been mentioned before, but what do I care?
The National Library of Scotland (blessings be upon them) has digitised the whole of the Ordnance Survey six-inch series maps. Thousands of 'em!
There's a choice of editions:
* Ordnance Survey, Six-inch to the mile, 1st edition - 1843-1882 - 2,123 sheets
* Ordnance Survey, Six-inch to the mile, 2nd and later editions - 1892-1960 - 7,486 map sheets
The latter is available as a seamless tapestry.
These are rather wonderful, because they show many features completely missing from the more recent 25,000 series maps.
e.g.
AveburyAnd the "British Settlements" that used to be on Fyfield and Marborough Downs. Presumably now robbed or ploughed out?
Or places like
Lerryn with the western end of The Giant's Hedge (drove road?)
Or ports like
Plymouth that were a triumph of Victorian railway engineering, with branch lines everywhere, down to every port and quayside.