There are a number of books available on-line, for free! Church-related ones are listed here.

United Kingdom/Republic of Ireland

Denominational links.

Howard Richter has drawn my attention to this huge on-line document, which lists all non-CoE places of worship currently licensed in England and Wales. Howard estimates about 32,000 in total. Very useful for when you are planning a photographic campaign! If anyone can point me to similar Irish or Scottish documents, I'll add those here as well.

It can be useful to search YouTube for videos of churches, and for photos, Geograph, The English Parish Church, The Digital Atlas, Bigblackcat, Flickr also has a church section, The British Library has a searchable archive, postcard sites: http://churchphotos.homestead.com/Britain.html; Postcard Traders Association.

Many individual counties have web-sites devoted to their churches - they can be found through the GENUKI site, which also has a concordance of the old and new counties. GENUKI itself also has many church photos.

For those planning a church visiting expedition, this website may prove useful - it lists the locked/unlocked/key-holder status of many churches - see the "Church Locking" tab. And Church Days is another website suggesting churches worth visiting.

Findachurch lists churches by selected area, with maps, and has links to many individual church web-sites.

Lots of useful architectural information on the Pevsner site. And the Corpus of Romanesque Sculpture in Britain and Ireland is a valuable resource too. For Catholic Churches, Taking Stock is devoted to assessing the "historical and architectural importance of every Catholic Church" in England and Wales.

Church Plans On-line has a large number of architects drawings.

The Church of England is compiling a database (CCEd) of clergymen from 1540 to 1835.

English Buildings is an architectural blog, including many of churches.

This is the Friends of Friendless Churches website.

A nice site devoted to "Tin Tabernacles".

British Listed Buildings - covering all of the listed properties in England, Scotland and Wales, many of which are, of course, churches.

Mike Berrell has pointed me to an interesting document listing dedications and their frequency. Sadly, the last page includes only a partial listing of dedications by parish, which would have been a very useful resource.

And for those possibly interested in buying a redundant church in England, here's a website to help - closed-churches-for-sale-england.html.

My Primitive Methodist Ancestors has many photographs of P.M. chapels - and also an Unknowns section.

The National Churches Trust.

The growing website http://www.oldukphotos.com has many church photos, and http://www.roll-of-honour.com/ is also worth checking out, although most photos are of commemorative monuments only.

Stock Photography of British and Irish Churches - Foto Search has hundreds of pictures of churches in Britain and Ireland, easily accessed by a search tool.

This website scans engravings from old out-of-copyright books. Many are of churches.

The website Old Pictures of the United Kingdom has photos from across the British Isles, some of churches.

The Footsteps website, although commercial, has a large collection of free-to-view church photos.

British Listed Buildings Online.

Historic Chapels Trust.

Stella Fisher has a particular interest in Saxon Churches. Her new website starts off with Yorkshire Saxon churches.

Mike Berrell recommends the British Churches Album, a website with similar aims to this one, http://www.derelictplaces.co.uk/, which includes some church photos, and a Flickr group, specialising in closed churches and chapels.
He has also drawn my attention to this book, which lists post-1914 churches listed by their architects.

Taking Stock is a website with many photos and historical notes about Catholic Churches in England and Wales.

Specialist interests - Medieval wall-paintings; Bell-ringing; Misericords; Stained glass, Holy Wells; Wool Churches, Yew Trees.

England

The English Heritage website contains many photos of listed buildings of all types, many being churches.
The Saints and Shrines of England.

South-East - http://www.roughwood.net/ChurchAlbum/ChurchFrames.htm specialises in church photos from East and West Sussex, and Kent.

Bedfordshire
http://www.countyviews.com/beds/church.htm

Bristol
Phil Draper's excellent http://www.churchcrawler.co.uk/

Buckinghamshire - most of the parish churches in Bucks are covered at http://www.countyviews.com/bucks/church.htm

Cambridgeshire - a fine site devoted to the churches of Cambridgeshire.  http://www.druidic.org/camchurch/

Cheshire - http://www.thornber.net:80/, http://www.moston.org/churches.html.

Cornwall
Paul Barnett recommends this site, and this one, which has a list of downloadable books relating to churches in Cornwall.

Cumbria - The Carlisle Diocese.
Ian Lewis has a blog which he describes as a "A gentle jaunt through a poignant aspect of Cumbria's history" - which is to say Cumbria's War Memorials.

Derbyshire - see http://www.derbyshireuk.net/derbyshirechurches_menu.html and http://www.derbyshirechurches.org.uk/.

Devon
A transcription of Some Old Devon Churches by John Stabb (1908-16), with many church photos, and a BBC article on a modern photographer who's following in his footsteps.
Bible Christian (Methodist) in North Devon - details on Genuki.
Howard Richter has advised of the Exmoor National Park website, which has a section on churches. Of particular interest is a discussion on the history of the "tin tabernacles".

Dorset - http://people.bath.ac.uk/lismd/dorset/churches/

Essex - a fine collection of photographs of Essex Churches. Also - Essex Church Photographs.

Greater London - for the City of London, see http://www.cityoflondonchurches.com/, and there is a more general London site here - http://londonchurchbuildings.wordpress.com/

Greater Manchester
Genuki has a number of photos here.

Hampshire - John Vigar's Hampshire site at www.hampshirechurches.co.uk. John's personal site is here.
Recommended by Ian Thirlwell is http://www.southernlife.org.uk/churchin.htm, and by Mike Berrell - http://www.hampshirechurchwindows.co.uk/

Hertfordshire - See Hertfordshire Church Photographs, and http://hertfordshirechurches.wordpress.com/

Isle of Man - See http://www.isle-of-man.com/manxnotebook/parishes/

Kent - John Vigar's Kent site at www.kentchurches.info. John's personal site is here.

Lancashire - Genuki has a number of photos here, as has the On Line Parish Clerks website.
See also http://www.lancashirechurches.co.uk/

Leicestershire and Rutland - see http://www.lrfhs.org.uk/
Also covering both counties is http://www.leicestershirechurches.co.uk/, which has many fine photos.

Lincolnshire - see http://www.wparkinson.com/, and http://lincolnshirechurches.blogspot.co.uk/.
A fascinating website looking at the graffiti in Lincolnshire churches.

Norfolk
http://www.norfolkchurches.co.uk/mainpage.htm

Nottinghamshire
http://www.oldnotts.co.uk/churches/index.htm and http://www.nottshistoricchurchtrust.org.uk/

Oxfordshire - see John Ward's site, www.oxfordshirechurches.com and Brian Curtis's http://www.oxfordshirechurches.info:80/

Rutland
Rutland Historic Churches Preservation Trust.

Shropshire - See John Cutts' site at http://www.shropshirehistory.co.uk/. The Shropshire FHS has a good number of church photos. Shropshire's non-conformist chapels at http://www.users.waitrose.com/~coxfamily/index.html

Somerset
Dave Bown has a website with many photos of the churches in the Sedgemoor area.

Staffordshire - http://www.thornber.net:80/

Suffolk - http://www.suffolkchurches.co.uk

Surrey - http://www.wsfhs.org/churches.htm

Sussex - http://www.sfhg.org.uk/church.html, and West Sussex - http://www.johnowensmith.co.uk/churches/indxwssx.htm

Warwickshire - Warwickshire Churches.

West Midlands - Many photos of the Birmingham Circuit Methodist Churches are here.

Worcestershire. Gilbert Barlow has a growing selection of Worcestershire churches on Flickr here.

Wiltshire - see http://www.oodwooc.co.uk/

Yorkshire
The Yorkshire Methodist circuits can be seen here - https://www.yorkshirenemethodist.org/district/circuits/

West Yorkshire
David Regan has pointed me to this West Yorkshire Archive Service page, though entries are given by name rather than location.

South Yorkshire
Heritage Inspired.
 

The Richard's Church Albums website contains many church photos, mainly from south-west England, and http://www.foxysislandwalks.co.uk/Oxfordshire-Church_Photos.html has many from Oxfordshire, Buckinghamshire and neighbouring Counties.
 

Isle of Man

A Manx Note Book - source for all things Manx, with many photos.

Northern Ireland

The Methodist Church In Ireland - Resources web-site includes photos of every Methodist Church in the island of Ireland.

County Tyrone - see Linda Bailey's site at http://freepages.genealogy.rootsweb.com/~tyrone/photos/churches/index.html

County Down
http://www.raymondscountydownwebsite.com/
http://freepages.genealogy.rootsweb.com/~rosdavies/CHURCHES/A-I.htm

Many churches on Jean McCarthy's website.
 

Republic of Ireland

The Methodist Church In Ireland - Resources web-site includes photos of every Methodist Church in the island of Ireland.

County Monaghan - see Brian McConnell's site at http://photos.yahoo.com/brian.mcconnell

Scotland

A changing set of featured churches from across Scotland on the Churches of Scotland site.

Some history of the Church of Scotland.

Argyll & Bute - Ian Phillips has a site specialising in Mull Genealogy, which has a collection of church photographs. http://www.mullgenealogy.co.uk

Dumfries & Galloway - See Sandy Pittendreigh's excellent site here.

Wales

Mike Berrell has advised that chapels in Wales are closing at the alarming rate of one per week. Recommended to Mike by the Deacon of Rhyl's Welsh Presbyterian Chapel, the Chapels Heritage Society celebrates their history and buildings. He also recommends the Grand Declines - Welsh Ruins & Welsh Ruined Mansions with many chapel photos, and the Royal Commission on the Ancient and Historical Monuments of Wales. Welsh Stained Glass in celebrated here.

Anglesey - is comprehensively covered on Phil Evans' Churches%20on%20Anglesey.htm. Also see the Anglesey - Mon Info Web.

Cardiganshire, Carmarthenshire and Pembrokeshire. See http://www.dyfedfhs.org.uk/.

Clwyd Family History Society has photos of every parish church.

Llyn Peninsula (Gwynedd) - tables of dates of Nonconformist chapels, with links to photos. http://www.penllyn.com/1/Hanes/capeli.html

Montgomery - Useful survey of Montgomery Churches (effectively, most of northern Powys).

Pembrokeshire
Video presentation of numerous churches and chapels.

Other Links

For those with an interest in church archaeology - http://www.cpat.demon.co.uk/projects/longer/churches/churches.htm

The Churches Conservation Trust.

The Ecclesiological Society, "for those who love churches".

English Church Architecture.

Gargoyles are the main subject of Robin Peel's latest venture.

Green Man - East Anglia explores "Green Man" carvings in the churches of Cambridgeshire, Lincolnshire, Norfolk and Suffolk, and their history and folklore. Many church photographs.

Historic Chapels Trust.

National Churches Trust.

UK Graves website contains many church photos.

A scholarly site dedicated to fonts in all their variety.