Many libraries and archives around the world, both brick-and-mortar and online, house collections useful for genealogy. In the United States, these varied repositories include, but are not limited to, the following.

Allen County Public Library Genealogy Center (Fort Wayne, Indiana)

Clayton Library Center for Genealogical Research (Houston, Texas)

Denver Public Library (Denver, Colorado)

Digital Public Library of America

Family History Library (Salt Lake City, Utah)

Google Books

Hathi Trust Digital Library

Internet Archive

Library of Congress (Washington, DC)

Mid-Continent Public Library Midwest Genealogy Center (Independence, Missouri)

National Archives and Records Administration (Washington, DC, and regional branches)

National Society Daughters of the American Revolution Library (Washington, DC)

Newberry Library, Genealogy and Local History (Chicago, Illinois)

New England Historic Genealogical Society Research Library (Boston, Massachusetts)

New York Public Library, Research (New York City, New York)

St. Louis County Library, History & Genealogy (St. Louis, Missouri)

Library of Virginia (Richmond, Virginia)

Wisconsin Historical Society Library