Note about free Social Security Death Index Access: In November and December of 2011, a number of genealogy sites removed or restricted their free SSDI access, access to the online version of the SSA Death Master File. The following sites still provide free SSDI access as of November 2012.
FamilySearch - SSDI Search
Free online search of the SSDI, a name index to deaths recorded by the Social Security Administration beginning in 1962. Free, unrestricted search. This database is generally current within a few months, but does not provide social security numbers for deaths occurring within the past 2–3 years.
Searching the Social Security Death Index (SSDI) in One Step
Steve Morse has created a very handy search form which enhances the search abilities of many of the free SSDI search engines on the Web. You can choose from a variety of free SSDI databases to search. This is the most flexible SSDI search interface available, and my favorite.
GenealogyBank - Free SSDI Search
Advanced search features make this free version of the SSDI easy to use, and it is updated almost weekly. However, results do not include social security numbers due to privacy concerns.
Mocavo - Free Search Social Security Death Index
Mocavo offers a free search of the Social Security Death Master File, current through about 2010, containing 88 million records. Results do include social security numbers.
Social Security Death Master File, free
Tom Alciere makes available this free version of the Social Security Death Master File, current as of November 2011, and searchable by name or social security number. This copy does not have available the death residence location or death benefit payout ZIP Code. For additional search features for accessing these files, check out the SSDI search tool at DonsList.net.
Social Security Death Index at Family Tree Legends
The SSDI database at Family Tree Legends is free, and offers a wide range of search features. There is no update schedule posted, however, and deaths from the last 10 years are not included.