OS 25 Inch Maps
The National Library of Scotland have digitised and geo-referenced a great number of old maps. Obviously much of their work covers Scotland but they have many old Ordnance Survey maps covering England on their web site, including the 25 inch England & Wales 1841 - 1952 Series, which are particularly useful for research purposes. A notable feature is that old and new maps can be displayed side by side or a slider can be moved to change the level of transparency to display modern mapping beneath the old.
Click on 'Map Images' (top left of screen) to explore the full range of maps on the site.