Use these Free Online Databases from any computer (no subscription required).
National Archives Databases
Search from many databases of records from around the world, including many provided from the National Archives. This website is a project of the Church of Jesus Christ of Latter-day Saints (Mormons).
Created by the Battery Conservancy from records at NARA, this free online searchable database contains entries for 10 million immigrants from 1830 through 1892, the year Ellis Island opened. Searchable by passenger name, the database provides information including age, sex, literacy, occupation, country of origin, port of embarkation, date of arrival into New York, and ship name. From 1855 - 1890, Castle Garden was America's first official immigration center.
Ellis Island has an online searchable database using records from NARA, (created by the Statue of Liberty/Ellis Island Foundation), of 22.5 million arrivals to New York between 1892 - 1924. Registration is required but free, and you can view scanned images of actual passenger manifests. You can also purchase copies through the site. Over 20 million immigrants passed through the Port of New York at Ellis Island from 1892 - 1924.
Electronic records at NARA relating to immigrants to America from Germany (1850-1897), Russia (1834-1897), Ireland (1846-1851), and Italy (1855-1900). The National Archives offers the AAD database as a free public resource and it can be accessed from anywhere.
New York Chinese Exclusion Index
The case files document the entry into the United States of Chinese aliens and the reentry of U.S. citizens of Chinese ancestry under the Chinese Exclusion Acts passed by Congress between 1882 and 1930, and repealed in 1943. The case files are available from the National Archives at New York City.