The Church of the Latter Day Saints
http://www.familysearch.org/
Most people should be aware of this site. But just in case you are not....
Includes various search options of various datasets listed below.
More information here :-
http://www.familysearch.org/Eng/Libr..._resources.asp
Watch out for errors.
Ancestral Files
Census 1880 - United States
Census 1881 - British Isles
census 1881 - Canada
International Genealogical Index
Pedigree Resource File
U.S. Social Security Death Index
Vital Records Index (Mexico and Scandanavia online) CDroms available for UK