Free sites ...
The London Gazette - historical archive from 1752, including Honours and Awards, bankrupcies, and complete coverage of the World Wars
British surname distribution - you enter a surname and can then see a maps of the UK showing the distribution of the surname in 1881 and 1998
Family search - this is the LDS site, a good source for parish records, particularly before 1837, and also provides a fully searchable transcription of the UK 1881 census
GenUKI - A great starting place for information across the UK, organised on a county by county basis.
Commonwealth War Graves - The "Debt of Honour Register" lists the 1.7 million from the Commonwealth forces who died during the two world wars plus details of the Commonwealth civilians who died from enemy action in the Second World War.
FreeBMD - an ongoing project with free access to volunteer transcriptions of the Civil Registration index of births, marriages and deaths for England and Wales. Definitely the first place to check, especially for 1930s and earlier.
FreeCEN - another volunteer site, providing a searchable database of UK census returns. Coverage varies but is the first place to check for the 1841, 1851, 1861, 1871 and 1891 censuses.
FreeReg - another volunteer site, providing a searchable database of UK parish baptism, marriage and burial records. They have now reached over 4.7 million, and more are being added.
UK BMD - provides links to over 400 web sites with transcriptions of UK births, marriages, deaths and censuses, transcribed by the local authorities in conjunction with Family History Societies, from the original indexes held by the local register offices
Societies ...
The Guild of One Name Studies - the premier UK-based organisation for one-name studies. Search their Register for free to see if any of your names are already being researched
Society of Genealogists - their library in London is well worth a visit, either as a member or for a modest day fee
Subscription and pay-per-view sites (with some free bits) ...
Ancestry 
- a subscription service providing searchable census transcriptions for 1841 to 1901 and also the birth, marriage and deaths index pages. Ancestry is available free from some public libraries and the census for 1841 to 1891 can be searched for free at the National Archives, Kew
Genes Reunited 
- based on the highly successful Friends Reunited, the site lets you post your family tree and search other's trees for common ancestors. Sites like this need a critical mass of subscribers to work but Genes Reunited seems to have achieved this. It's free to search but you must subscribe to contact others
Findmypast - formerly 1837.com, a pay-per-view or subscription site with images of the English and Welsh Civil Registration birth, marriage and death indexes from 1937 to 2002. The site also has a complete transcription of the 1841 to 1911 England and Wales census
The Genealogist - a subscription or pay-per-view service with census 1841-1901, BMD from 1837-2005, parish records and much, much more
Origins Network - a rich source of records not found elsewhere, organised by British Origins, Irish Origins and Scots Origins
Scotlands People - one of the largest online sources of original genealogical information with over 43 million records to access covering statutory and parish registers, census returns and wills
1901 Census online - the original 1901 online census, now hosted by Genes Reunited. You have to pay to see the full details but you can get a lot of information without paying by using the advanced options of the free search facility
More free sites ...
Old Maps - provides free access to Britain's most extensive digital historical map archive. Use it to find out where your ancestors used to live or check out what your neighbourhood looked like over a hundred years ago
Parish Finder - UK Parish Database - a very simple but very powerful lookup for English parishes which also lists neighbouring parishes and distances apart. Invaluable to track down parish records in Victorian times and ancestors who married in a nearby parish.
Charles Booth online archive - a free searchable resource giving access to archive material containing the original records from Booth's survey into life and labour in London, dating from 1886-1903 and Booth family papers from 1799 to 1967
Surnames frequency lookup - this database, extracted from Office of National Statistics data, contains a list of surnames in use in England and Wales in Sept 2002, with almost 270,000 surnames, shared by 54.4 million people. Search it for free to find the frequency of any surname.
Cyndis List - They claim more than 250,000 genealogy links covering the entire world.
A Vision Of Britain - a vision of Britain between 1801 and 2001 including maps, statistical trends and historical descriptions