The Church of Jesus Christ of Latter-day Saints (the Mormons), this site contains a large and freely searchable archive. Among the data are: the Social Security Death Index (SSDI), U.S. Federal Census for 1880 or the 1881 census for both the British Isles and Canada, a vital records index containing birth, marriage, and death records from around the world.
The USGenWeb Project consists of a group of volunteers working together to provide Internet websites for genealogical research in every county and every state of the United States. Organization is by county and state, and this website provides you with links to all the state websites which, in turn, provide gateways to the counties. For broader perspective, you can also look into WorldGenWeb.