Including information on soldiers from other parts of the British Empire
The Long Long Trail www.longlongtrail.co.uk
The best site on the First World War British Army. Contains no actual records but explains what they all are, how to find and understand them and has links to other useful sites. It also has the most complete list of surviving absent voter rolls available.
The International Red Cross grandeguerre.icrc.org
Information on men captured and missing, though difficult to search.
Army Lists of Officers digital.nls.uk/british-military-lists/archives/88735803 Canadian service records www.bac-lac.gc.ca/eng/discover/military-heritage/first-world-war/Pages/introduction.aspx
War Diaries and service records.
Australian War Memorial awm.gov.au/about/collection/awm4/
Histories, rolls and diaries for Australian units.
Australia and New Zealand discoveringanzacs.naa.gov.au
Free digitised WW1 service records for Australia and NZ.
Indian Army searcharchives.bl.uk
Records for British soldiers in the Indian Army are held at the British Library. Indexed but not digitised.