The Social Security Death Index Sponsored by rootsweb.com. Lists all deceased individuals who collected from the Social Security administration: their dates of birth and month/year of death and residence(s). Try the advanced search option.
Rootsweb.com A great website, with links to many family trees online. Has an excellent global search engine that lets you search online family trees when you only have tiny fragments of information, such as first name and date of birth only. Free.
FamilySearch.org Run by the LDS church (Mormons), has the 1880 Census fully indexed for every person, not just head of household. Difficult to use if you don't have first names; cannot be used at all without last names. Excellent resource. Free.
CyndisList.com The best collection of links to genealogical websites available. Excellent, but a little overwhelming.
Genealogy.com Has the advantage of having the 1900 and 1910 censuses indexed, which Ancestry.com does not. Also has indexed census records for 1790-1820, 1870 and part of 1890. Good resource, but a pay site. Offers a monthly subscription.
Ancestry.com The grand-daddy of on-line genealogy database sites, but can be expensive. Does offer a free trial. Has the 1920 and 1930 censuses indexed, as well as all census from 1790 to 1850.