Some of the websites that I have found useful are:
The Commonwealth War Graves Commission: http://www.cwgc.org/
Listing of Births, Marriages and Deaths in the UK: http://freebmd.rootsweb.com/
Far Eastern Prisoners of War: http://www.fepow-community.org.uk/
The website for Army Museums is: http://www.armymuseums.org.uk/
The UK National Inventory of War Memorials: http://www.ukniwm.org.uk/
The Long, Long Trail. The British Army in the First World War: http://www.1914-1918.net/index.html
The Australian Virtual War Memorial: http://www.awm.gov.au/
Canadian Archives: http://www.collectionscanada.gc.ca/military-peace/index-e.html#databases
New Zealand War information: http://muse.aucklandmuseum.com/databases/cenotaph/locations.aspx
Scottish National War Memorial: http://www.snwm.org/website/frames.html
Portsmouth Naval Memorial: http://www.memorials.inportsmouth.co.uk/southsea/naval.htm
The Royal Naval Patrol Service Memorial, Lowestoft:
http://www.rnps.lowestoft.org.uk/memorial/memorial.htm
Neuve Chapelle, the Indian Memorial:
http://www.cwgc.org/find-a-cemetery/cemetery/144000/NEUVE-CHAPELLE%20MEMORIAL
The Guild of Church Bell Ringers: http://www.cccbr.org.uk/rolls/casualties/?warID=1
For Chaplains: https://sites.google.com/site/chaplainsatwar/home
Personally, I find that the most useful site, but you have to pay to use it, is: http://www.ancestry.co.uk/
Listing of Births, Marriages and Deaths in the UK: http://freebmd.rootsweb.com/
Far Eastern Prisoners of War: http://www.fepow-community.org.uk/
The website for Army Museums is: http://www.armymuseums.org.uk/
The UK National Inventory of War Memorials: http://www.ukniwm.org.uk/
The Long, Long Trail. The British Army in the First World War: http://www.1914-1918.net/index.html
The Australian Virtual War Memorial: http://www.awm.gov.au/
Canadian Archives: http://www.collectionscanada.gc.ca/military-peace/index-e.html#databases
New Zealand War information: http://muse.aucklandmuseum.com/databases/cenotaph/locations.aspx
Scottish National War Memorial: http://www.snwm.org/website/frames.html
Portsmouth Naval Memorial: http://www.memorials.inportsmouth.co.uk/southsea/naval.htm
The Royal Naval Patrol Service Memorial, Lowestoft:
http://www.rnps.lowestoft.org.uk/memorial/memorial.htm
Neuve Chapelle, the Indian Memorial:
http://www.cwgc.org/find-a-cemetery/cemetery/144000/NEUVE-CHAPELLE%20MEMORIAL
The Guild of Church Bell Ringers: http://www.cccbr.org.uk/rolls/casualties/?warID=1
For Chaplains: https://sites.google.com/site/chaplainsatwar/home
Personally, I find that the most useful site, but you have to pay to use it, is: http://www.ancestry.co.uk/