UK Census - Tracing Your Ancestors in UK Census Records
Ancestry.co.uk - Census Collections
Ancestry.co.uk has digitized and placed online the complete collection of available national census records for England, Wales, the Channel Islands and Isle of Man for the years 1841-1901. They also have the Ireland 1766 Religious Census.
Subscription Required for Access
National Archives - 1901 Census for England & Wales
You can search the online 1901 England and Wales census for free on this site operated by the National Archives, but you have to pay to access transcriptions and digital images of the actual census records.
This subscription-based genealogy site offers a wealth of records for England and Wales, including the 1861 and 1891 census. Their indexes are more complete than most other online offerings.
Subscription & Pay-Per-View
Volunteers worldwide are transcribing UK census records for 1841, 1851, 1861, 1871 and 1891. Coverage varies widely - with some counties and years almost 100% complete, and other counties with nothing.
Online access to the Scottish census indexes for 1841, 1851, 1861, 1871, 1881, 1891 and 1901 and images of the 1841, 1851, 1861, 1871, 1891 and 1901 census returns are available on ScotlandsPeople. You can also view transcripts of the 1881 census.