On March 3, I published an announcement from the National Archives and Records Administration (NARA). The announcement at http://blog.eogn.com/eastmans_online_genealogy/2008/03/nara-makes-some.html describes the 5.2 million records of some passengers who arrived during the last half of the 19th century at the ports of Baltimore, Boston, New Orleans, New York, and Philadelphia. These records have now been placed online for the first time.
The records can be accessed through NARA’s online Access to Archival Databases (AAD). The records include the name of the ship, the port, and the date. As several commenters pointed out after the announcement was published, the process of finding all that information is not intuitive, especially the use of the Manifest File. Now Steve Morse of "One-Step Genealogy" fame has created a trio of One-Step tools to simplify the use of the NARA databases.
In describing the new tools, Steve Morse wrote, "From the results that I present, it's very obvious how to get the ship details. Rather than displaying a number corresponding to the manifest ID as they do, I instead display a link that says "get ship". I also fixed up some other rough spots that appear on their site, such as the CLEAR button coming to the left of the SEARCH button (what in the world were they thinking?)."
The tools are at the bottom of the OTHER PORTS section of Steve Morse's "One-Step Portal for On-Line Genealogy" at http://stevemorse.org.