This will be a (growing!) list of websites that I find useful.

Really useful sites for every genealogist
FamilySearch maps
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

Useful for Leeds
Leodis – a fantastic collection of photographs of old Leeds
Martin & Tallis map of Leeds, 1851 – from the David Rumsey Map Collection site
The streets of Leeds in 1821 – from GENUKI.  Really helpful to read through for when you can’t read the street name on an old record.

Useful for York
Captain Tucker R.E. map of York – surveyed between 1849 and 1851 for the Ordnance Map Office. Large scale, interactive – you can choose to include the current road layout as an overlay.

Useful for Norwich
George Plunkett – a fantastic collection of old photographs and maps of Norwich and Norfolk
Norwich Yards – much of Norwich was laid out as medieval yards leading off the main roads. Search for them here, also photos and other useful period maps.

Useful for London
Tallis’s illustrated plan of London and its environs, 1851 – from the David Rumsey Map Collection site

DNA blogs and info
DNA explained

Regional Family History Societies
And other websites specialising in English regions and other parts of the British Isles.
If the one you need isn’t listed here, just Google to see what exists.

Yorkshire Roots
Thoresby Society – a very well-respected local history society for Leeds
York Family History
Midland Ancestors
Norfolk Family History Society
North of Ireland Family History Society
IrishGenealogy.ie