It’s now possible to find all of the publicly available UK census online. You will only be find records from 1841-1911 as there is a law which protects census records for 100 years.
The best websites for finding and viewing these records are:
Find My Past – FMP is my personal favourite for pretty much all UK genealogy records. Searching is easy and you get relevent matches to your queries. It’s also possible to download images of census returns so you can save them on your computer for future reference. FMP has the England, Scotland & Wales census records from 1841-1901, plus England & Wales from 1911.
1939 Register – Although not strictly a census, the 1939 Register is almost as good as.
The Genealogist – Another great website for the England and Wales census from 1841-1911.
Genes Reunited – I started my own family history research on this website. They have great coverage of the UK census, plus you can build your family tree online. There’s also a community where you can ask questions about your research and get quick answers.
Scotland’s People – As far as I know, this is the only place you can search and view the complete 1841-1911 Scottish census online.
Census records for lovers of all things free
FreeCEN – Yes, it’s really free. It includes records from 1841-1891 (England, Wales and Scotland).
FamilySearch – There are various census records available for free here, but you won’t be able to download any images. It’s still a very useful tool.
Census of Ireland 1901/1911 – The National Archives of Ireland have made available the 1901 & 1911 Irish census free of charge.
Graham Maxwell – Some free Scottish censuses from 1841-1861