Information Sources - UK General

IGI/FamilySearch - http://www.familysearch.org
(includes a lot of Births/Marriages info, mainly up to around mid-19thC, and the 1881 census for England & Wales)

FreeCEN - http://www.freecen.org.uk/
(project which is gradually making census data available for free - patchy at present but more coming online all the time)

Old-Maps.co.uk - http://www.old-maps.co.uk/
(excellent for finding places now long gone)

GENUKI (UK & Ireland Genealogy) - http://www.genuki.org.uk/