Library & Archives Canada
Before the arrival of the Loyalists and British military settlers, present-day Ontario was part of the province of Quebec. Following the Constitutional Act of 1791, Quebec was divided to create Upper Canada (Ontario) and Lower Canada (Quebec). Many early settlers, both military and civilian, submitted petitions to the Governor to obtain Crown land. Sons and daughters of Loyalists were also entitled to free land.
With this online Upper Canada Land Petitions (1763-1865) database, researchers can access 77,000+ references to petitions for grants or leases of land by individuals who lived in present-day Ontario.
Once the file information has been located from the above site, the original petitions can be viewed. Unfortunately, they are not indexed, requiring some browsing.
While on a Loyalist bent, the Sir Guy Carleton Branch of the United Empire Loyalists' Association of Canada (UELAC) has the Carleton Papers (1774-1783) online.
The index can be used to find ancestors who passed through New York City during the Revolution, ancestors who fled or were evacuated, ancestors sent to Canada under specific orders, ancestors who were soldiers of a British Regiment demobilized in Canada, ancestors who were soldiers of a German Regiment, 'rebels' who: wrote letters to Headquarters, whose property was confiscated, or who were imprisoned.
McGill University has digitized its collection of Ontario's County Atlases (1874-1918). Researchers have long recognized the importance of the increasingly-scarce print versions. McGill's Rare Books and Special Collections Division is fortunate in owning copies of forty-three original atlases.
The County Atlas Digital Project is a searchable database of the property owners' names which appear on the township maps in the county atlases. Township maps, portraits and properties have been scanned, with links from the property owners' names.
The Wesleyan Methodist baptismal records (1820+) from the United Church Archives in Toronto
The Archives of Ontario website
The Ontario (Upper Canada) Genealogy site, maintained by Michael Stephenson
Tax assessment rolls provide a property's physical address, description, and year of building construction. With this information, one can research the history of a property at the Port Hope Archives, where the land registry books (Crown grants to 1955) and deeds (1867-1955) are located. Any information later than 1955 can be found at the Land Registry Office at Sir Sandford Fleming College, Cobourg.
Ontario ancestors can sometimes be found in New York's Ellis Island records.
FamilySearch, the genealogy site of the Latter Day Saints
A useful calendar site