Census

Census for 1851, 1861, 1871, 1881, 1891 and 1901 online
Details about and links to the 1851, 1861, 1871, 1881, 1891 and 1901 censuses available online
Online searchable database for 19 Century census
A project aiming to provide a "free-to-view" online searchable database of the 19th century UK census returns.
Gendocs website for researching Victorian London
Help with researching Victorian London, particularly useful in connection with the 1841-1891 censuses.
Kindred Konnections
Includes the 1871 Cornwall census
The National Archives of Ireland
The National Archives of Ireland holds what remains of the original census returns. This includes the 1911 census returns. The website lists what material is available to researchers.
Public Record Office of Northern Ireland
Unfortunately, very little survives of the Irish censuses. PRONI holds microfiche copies of the surviving material for Northern Ireland up to and including 1901. The website also lists alternative sources of information.
ScotlandsPeople
A website run by Scotland Online on behalf of the General Register Office for Scotland. A pay-per-view website including indexed digital images of the statutory registers of births (1855-1903), marriages (1855-1928) and deaths (1855-1953) for Scotland, the 1891 and 1901 Scottish censuses, and indexes to the 1881 census and the Old Parish Registers of baptisms and marriages.
Census Online
This site links to thousands of census transcription sites world-wide. The webmaster of census-online, Mark Reed, transcribed the 402-page 1850 Greene County, Tennessee census, and began keeping a list of other online census transcriptions. Since 1996, that original list of 30 links has grown to over 17,000 links.
Family Search
This site provides a good starting point for family history research.