The following websites are excellent resources on a variety of historical topics. Included in this list are sites that offer access to primary source materials, and also those that provide general summaries of historical periods and events. This list is not comprehensive, and you may find other websites that are useful.
Internet Archive is a non-profit library of millions of free books, movies, software, music, websites, and more.
Making of America, created and maintained by Cornell University Library, is a digital collection of monographs and journals documenting American social history.
The Massachusetts Historical Society is a nationally recognized research library with archival collections pertaining to major events and figures in the history of the United States. MHS's online collections include the personal papers of John Adams and Thomas Jefferson, documents on the Civil War and the end of slavery, maps from the French and Indian War, and more.
The official site for the National Archives and Records Administration indexes digital copies of U.S. historical documents, including the Declaration of Independence, the Bill of Rights, and World War II photographs.
A gateway to state digital collections from the Oklahoma Department of Libraries.
Salem Witch Trials: Documentary Archive and Transcription Project is a digital library of court records, letters, diaries, and maps pertaining to the Salem Witch trials. All of the site's materials are freely accessible to the public, courtesy of the University of Virginia Libraries.
The Valley of the Shadow is a digital library of letters, journals, government records, and newspapers documenting the Civil War in Augusta County, Virginia, and Franklin County, Pennsylvania. This digital library is made available by the Virginia Center for Digital History at the University of Virginia.