Thanks to Dick Eastman for mentioning this on his blog.
NARA has added indexes to some pre-1900 passenger lists to its database online. The search is not very user friendly… names and ship information require different searches. So you have to locate the person first and then go to the ship search to get the full details.
You will find the passenger list indexes here. Be sure to read the description of each one.
Also see the explanation on how to search at the Genealogy Insider blog here.