The most important online collection for genealogists is the Chronicling America, Historic American Digital Newspaper Project with over 11 million pages of freely accessible and searchable newspapers from around the United States. I have found this to be a valuable source for information about my own family members. You can search the entire collection or focus on one state or county with an advanced search. There is also a searchable list of most of the newspapers published in America since the 1600s. The list shows where and when each newspaper was published and repositories where copies are available.
This list of newspapers, called the U.S. Newspapers Directory, 1690 - Present, illustrates the fact that many researchers fail to use the resources that are available to find their ancestors. If you do a search in a specific county, you will likely be surprised at the number of newspapers that were printed during the time your family was living there.
You can also obtain items from the Library of Congress through interlibrary loan. But the borrowed items must be read or used in a local library; they will not be allowed to be checked out.
The Library of Congress also has an extensive online map collection and another extensive collection of photographs. Although not of primary interest to genealogists, the Library as a lot of video and audio files online. You may also benefit from the Preservation section of the Library. They have extensive resources concerning the preservation of all kinds of documents and artifacts.