Library and Archives Canada: A good jumping-in point, it has a large and growing collection of wartime records. For example, its index of Canadians involved in the First World War often includes a soldier's signature, next of kin, a physical description and in some cases a photo.
Project 44 is an online map of the western front during WW2 and includes maps, war diaries and is continually digitiizing more war diaries or that theatre
Canadian Virtual War Memorial: An extensive database of the location of military graves, and an option to order a copy of a specific soldier's entry in the Book of Remembrance. which is maintained by the Dept. of Veterans Affairs and dedicated to those Canadians who died in all wars in service to their country.
Commonwealth War Graves Commission: Personal and service details and places of commemoration for the 1.7 million members of the Commonwealth forces who died in the First or Second World Wars.
FindAGrave.com: Not limited to the military, it can help locate the gravesites of specific people and has photos of cemeteries and graves.
Canadian Post War Military and Dependent Graves: Burial locations of service members and their dependents who died outside of Canada and the USA but were not returned to Canada.
Valour Canada Extensive records on Canadian military history, anecdotes,, etc
Cyndi's List: This site gathers lists of links to useful genealogy research sites. There's an extensive list of military sites, databases and help forums.
FamilySearch.org: A jump-off point to multiple databases and resources. For guides to finding military information, follow the "Learn" link at the top of the page, click on "Research Courses," and choose "Military" from the left-hand column.
Canadian Merchant Navy An index of merchant sailors and their ships.
WorldVitalRecords.com: A collection of everything from personal information to tombstone photos.
AutomatedGenealogy.com: Links to census information and wartime databases.
Genealogylinks.net: A collection of links to smaller Canadian military archive sites.
FindMyPast.co.uk: This United Kingdom-centric site includes military records pertaining to other countries.
Odessa3.org: A genealogical library drawing on sources of information about people of German and Russian descent
Good background on the First World War
http://www.1914-1918-online.net/06_WWI_websites/index.html list of ww1 websites.
International Encyclopedia of the First World War
https://encyclopedia.1914-1918-online.net/home/
Eyewitness to History - first hand accounts of key events in the war
Medals - for sale and for info
Marway Militaria - Winnepeg source for medals and info
Army Surplus World - medals and other items
Newspapers
Canada
BC Historical Newspapers UBC - BC newspaper archives - free
Peel.library University of Alberta - Prairie province newspaper archives - free
www.ancestry.com/search/rectype/periodicals/news/main.htm
www.godfrey.org – Godfrey memorial library.
Britain
http://www.britishnewspaperarchive.co.uk - British newspapers
https://www.nls.uk/collections/newspapers - This is the National Library of Scotland newspaper site.
https://www.thegazette.co.uk/ - The London Gazette newspaper archives. http://catalogue.nrscotland.gov.uk/nrsonlinecatalogue/welcome.aspx
International bomber command losses database crews, info on deaths
Military History
Library and Archives Canada Military Personnel records.
Gathering our Heros - a compendium of Chatham Kent War vets
https://www.forces-war-records.co.uk/
http://www.irishwarmemorials.ie/
Canadian Great War Project - A member/reader project to identify and tell the story of Canadian Veterans of WW1
http://ww1.canada.com/memory-project
https://www.cwgc.org/ - Commonwealth War Graves Commission.
http: //www.greatwar.co.uk/research/family-history/trace-ww1-british-soldier.htm
http://www.608dukes.com/uploads/8/5/5/1/8551447/cf_manual_of_abbreviations.pdf - miliary abbreviations.
https://army.ca/wiki/index.php/Canadian_Military_Acronyms - Canadian Military Abbreviations
http://www.inflandersfields.be/en - Flanders Field
https://www.familytreemagazine.com/premium/ww1-genealogy-websites/ - lists good military sites and gives information about each.
http://www.canadaatwar.ca/forums/
http://1914-1918.invisionzone.com/forums/ - the Great War forum – the best
https://www.thegazette.co.uk/ - official newspaper for war reports
http://www.roll-of-honour.com/ - British site
http://www.iwm.org.uk/memorials/search?utm_source=ukniwm&utm_medium=rw&utm_campaign=wmalaunch
http://www.1914-1918.net/links.htm
http://www.militaryhistoryonline.com/forums/ - good source for American battles from the Civil war on.
http://scottishmilitaryresearch.co.uk/ - Scottish Military information
https://mspace.lib.umanitoba.ca/bitstream/handle/1993/5271/Abra_Glennis.pdf?sequence=1 #5 General Hospital history
http://onlinebooks.library.upenn.edu/webbin/serial?id=cdnnurse
http://www.militaryhistoryonline.com/forums/ - good source for American battles from the Civil war on.
http://scottishmilitaryresearch.co.uk/ - Scottish Military information
http://onlinebooks.library.upenn.edu/webbin/serial?id=cdnnurse – Canadian nurse magazine
https://www.wrecksite.eu/wreck.aspx?96349 – website with shipping wrecks and ships registries )
http://www.convoyweb.org.uk – the convoys of WW2
http://www.daileyint.com/seawar/apfjtwas.htm - santa elena blogs and stories etc.
USA
American Civil War Website provided by the National Park System - Soldiers and Sailors Database
American Battle Monuments Commission - US WWI and WWII burials
Civil War soldiers and sailors database - National park service database.