Using the Social Security Death Index can advance your genealogy.
It contains information on over eighty million deceased individuals with United States social security numbers whose deaths were reported to the Social Security Administration. Birth years for the individuals listed range from 1875 to last year. Information in these records includes name, birth date, death date, and last known residence.
When you locate the individual for whom you are searching, you can generate a letter to the Social Security Administration you can send along with payment for a copy of the original application your ancestor filled out. The application shows the address where they were living at the time, along with their parents' names (including mother's maiden name). These documents are very valuable to genealogists.
The Rootsweb version of the Social Security Death Index includes a full tutorial on using it.