National Archives’ Resources for Genealogists. Different branches host different records (e.g., the Boston branch in Waltham, MA, houses citizenship records for New England. NARA also has the institution edition to Ancestry U.S. Discovery and Ancestry World Explorer for free use.
An excellent place to start. “A categorized & cross-referenced index to genealogical resources on the Internet.” For Italian resources, use the listing under ‘Genealogy Categories.’
Searches the Ellis Island database for ships arriving into the Port of New York at Ellis Island from 1892-1954. Requires a free account.
Searches the Ellis Island database using a different search engine. Use the Ellis Island Passengers Gold Form (1892-1924). Useful for locating ship manifests that may not be indexed on Ancestry. You can then make a note of the information and manually search the rolls on Ancestry. Also useful for searching just by surname or just by town (comune). Requires a free account on http://www.EllisIsland.org.
Searches the ships arriving into the Port of New York at Castle Garden from 1820-1892.
New England Historic Genealogical Society. Italian books and articles of interest. A wide range of databases including Boston Archdiocese