A census has been taken in what is now Ontario since the early 1800s, but few of the early ones have survived. An index of all surviving Canadian censuses on microfilm is available on the Library and Archives Canada (LAC) website. Head of Household censuses exist for 1842, 1848, and 1850, and there are indexes for 1848 and 1850. These are available on microfilm through Family History Centers.
From 1851, there are decennial Nominal (i.e. everyone is listed) censuses. The 1851, 1861, 1871, 1881, 1891, 1901, and 1911 are widely available on microfilm (Family History Centers, or interlibrary loan). Many Ontario libraries and archives have the films for their area.
A Head of Household and strays (people not related to the head) index for the 1871 census was done by the Ontario Genealogical Society and is available on the LAC website. There is also an online Canadian Census of Industrial Establishments, 1871, which lists over 45,000 industrial establishements.
There is a full index for the 1881 census on the Family History website.
The 1901 census can be searched by geographic location (not by name) on the LAC website. Automated Genealogy has produced a name index for the entire Canadian 1901 census, but it is not fully proofed. The 1901 census contains much more information than the earlier ones.
The LAC site has a geographic search for the 1911 census and Automated Genealogy has developed a name index. The 1916 Census of the Western Provinces (Manitoba, Saskatchewan and Alberta) has been released.
The 1921 census was released in August 2013. ndexing will take some tije, but the complete census images are available for browsing.
If you wish to consult a non-Canadian census, refer to a guide for that area. A census document is more complex than most people realize! There are several sources for censuses in other countries, including AllCensusRecords.com.