The SSDI, taken from the U.S. Social Security Administration's Death Master File, contains those individuals who were assigned Social Security numbers and whose death was reported to the SSA. Few records before 1964 have been digitized, and thus most records date from 1965 and later. The database is updated monthly.
There are several sites that provide access to the SSDI, among them NEHGS, Ancestry and SSDI One-Step. While some of these sites have a membership class, all of these provide free access to SSDI. Depending upon the particular search interface, the SSDI mirrors may be searched by name, birth or death year or month, last residence, and social security number.
SSDI One-Step is actually a search interface providing capabilities to access nine different search engines individually, including the NEHGS, Ancestry and RootsWeb resources.