GENUKI – Genealogy UK and Ireland
David Rumsey Map Collection – great map resource. Best if you use the advanced search
British Ceremonial and Historic Counties Maps – useful for zooming in and finding the exact historic county boundaries – particularly useful for London which was formerly divided into different counties.
National Library of Scotland Side by Side Maps
Registration Districts in England & Wales (1888)
Yorkshire wapentakes – the wapentake is the Danelaw equivalent of the Anglo-Saxon hundred.
Websites providing online maps