This, by no means, is exhaustive. Please leave comments and recommendations for more sites.
Start Here: Researching Canadian Soldiers of the First World War by Michael O’Leary of the Regimental Rogue
This resource is absolutely a must read. At the very least a review of the sections will help you understand the resources available to use online to research soldiers and units of interest. Some of the screen-shots of the sites are out of date and there may be some links that do not work but DO NOT let that dissuade you. This is the best place to start.
Library and Archives Canada Soldier Search Page
This resource gives you access to the soldiers who served in the Canadian Forces in the First World War. The search interface is basic but the LAC is slowly digitizing the service records of the soldiers in this database and that is an incredible resource.
Library and Archives Canada Circumstances of Death Registers
This resource is a basic repository of the cards recording the circumstances of death. It is NOT searchable and can be a challenge to find your soldier as it is in alphabetical order.
Canadian Great War Project
An amazing resource. Status of updates and maintenance is not known but the soldiers database is mostly soldiers that died during or just after the war with links, in this case, directly to the Canadian Virtual War Memorial and C.W.G.C. web page corresponding to that soldier.
Resources for a whole range of military subjects pertaining to the Great War and Canadian Forces and soldiers.
Canadian Great War Project: Master List of Battalion Diaries
List of battalions with links to the LAC digital depository of war diaries. This link is focused on battalions only.
Canadian Great War Project: Master List of all Unit Type Diaries
List of all other unit types leading to pages pertaining to such units.
Veterans Affairs: Canadian Virtual War Memorial
Veteran Affairs has a database of all the service personnel lost during service, in peace and in war, from the First World War until present day. Many people have contributed images relating to the soldiers that can be valuable such as newspaper clippings.
C.W.G.C. database of ALL Commonwealth soldiers buried in military cemeteries and other locations throughout the world. Many of the W.W. 1 cemeteries are named after place names used in the War Diaries. Digital documents with information pertaining to burial information.
18th Battalion War Diaries: April 1915 to February 1918
Links specifically to the 18th Battalion War Diaries.
The Gazette: Awards and Accreditation Search
The Gazette is a MASSIVE repository of social historical information. It is so dense with information it is highly recommended to review the link below before stepping into this treasure trove of information.