Society of Genealogists: www.sog.org.uk
Every serious genealogist should be a member - the records are very extensive indeed.
Family Records: www.familyrecords.gov.uk
Helpful introduction to the subject.
The National Archives: www.nationalarchives.gov.uk
Also a helpful introduction to the subject.
ScotlandsPeople: www.scotlandspeople.gov.uk
Official Scottish records.
FamilySearch: www.familysearch.org
Church of Jesus Christ of Latter-Day Saints records.
Commonwealth War Graves Commission: www.cwgc.org
Register of First and Second World War burials.
Ancestry: www.ancestry.co.uk
Access to, among other things, the 1861, 1871, 1881, and 1891 Censuses.
College of Arms: www.college-of-arms.gov.uk
Grants of Arms, Visitations and Pedigrees.
FreeBMD: http://freebmd.rootsweb.com
Index of English births, marriages and deaths from 1837 to c. 1910.