For American historians, the Library of Congress website is the place to start. The Library's National Digital Library Program, the "American Memory Project", began in 1989 and now includes numerous major Web-based collections. The depth, range, and diversity of these online collections dwarf anything else available for American historians on the web, and include material in multiple media (books, manuscripts, films, and sound recordings), but especially photographs; they contain about 70,000 images in eight different collections, from nineteenth-century daguerreotypes to color photos taken by the Farm Security Administration and Office of War Information in the 1930s and 1940s.
They also include multiple perspectives--dissidents are well represented in the Woman Suffrage and African American pamphlet collections, whereas such establishment figures as John D. Rockefeller literally have their say in the Nation's Forum sound collections. The Founding Fathers make their appearance in 274 broadsides from the Continental Congress and the Constitutional Convention, while hundreds of unfamous Americans tell their stories in the WPA life histories.
The National Archives and Record Administration also offers a substantial online collection, including finding aids and an online exhibit hall.
Yahoo! maintains a useful list of foreign libraries and archives which maintain web sites.
Students should also consult the web page put together by UTSA reference librarian and history bibliographer Richard McDonnell. This web site offers detailed information on the library's holdings of primary and secondary sources, in both print and electronic format, which will be of interest to UTSA history students.