Ancestry.Com
Available by subscription or for free when accessed from many libraries. It contains every-name index of every available US census from 1790 to 1940, as well as extensive passenger lists and many local records. For Jewish records, see Jewish Family History Collection
Genealogy.Com
Available by subscription or for free when accessed from many libraries. It contains every-name index of every available US census from 1790 to 1930, as well as extensive passenger lists and many local records.
JewishGen– The largest and most comprehensive website for Jewish genealogy – research information, tools, and extensive databases. One must register to access some sites, but there is no fee.