George Mason University - History Matters: The U.S. Survey Course on the Web
http://historymatters.gmu.edu/
This site offers a very rich and entirely searchable or browsable collection of digital primary source archives available through the open web. Go to the Advanced Search page to browse for search options, or type in keywords that represent your topic to get a list of reputable and interesting collections on the open web.
Presidential libraries offer searchable lists of archived historical materials available through the library, and sometimes through the open web.
John F. Kennedy's Presidential Library
http://www.jfklibrary.org/Historical+Resources/
Richard Nixon's Presidential Library
http://www.nixonlibrary.gov/forresearchers/learning.php
Or, search for other presidential libraries that might be helpful to you through the National Archives:
Selected primary source documents from 1991 to present covering: Business, Economy, & Work; Defense, Security, & Military Affairs; International Affairs; Media, Culture, & Life in America; Health Care, Social Services, Housing, and Education, etc.
Go the the 'Legal' tab when you enter this database to search through Supreme Court case transcripts, decisions and opinions. Do note that the default for searching is both federal and state cases - you may want to choose one or the other for your search.
U.S. Department of State - Office of the Historian
This site offers searchable collections (sometimes in full-text format, other times only in bibliographic format) of historical documents pertaining to foreign relations in the United States during the Kennedy, up to the Ford administration. Go to the 'Historical Documents' page to browse the resources by theme, by chronology, or by volumes, as they were published.
Digital National Security Archive
http://nsarchive.chadwyck.com/home.do
The Digital National Security Archive contains the most comprehensive set of declassified government documents available resource now includes 36 collections consisting over 80,000 meticulously indexed with more than 500,000 total pages.
The following collections were added to the SJU subscription in the fall term 2011:
University of California - Santa Barbara - The American Presidency Project
http://www.presidency.ucsb.edu/index.php
Search or browse through transcripts of inaugural addresses, state of the nation speeches, press conferences, as well as public orders, proclomations, signing statments, etc. through this site. Go to Documents to browse by term and president.
National Archives
The portal into the U.S.'s National Archives, click on 'Start your Research' to investigate research paths and see what they make available online. Also try:
The Digital Vaults, supported by the Foundation of the National Archives
A flashy but very useful site that allows you to browse through or search for historical documents that span across U.S. history. This is a project to "open the stacks" of the National Archive, making all these documents digitally accessible.
The Library of Congress American Memory Collection
http://lcweb2.loc.gov/amhome.html
This site offers a rich collection of over 7 million American primary historical documents dating back to the 15th century. Browse to find your topic, or do a very specific search, and then use the subject headings you find in relevant documents - they will be hyperlinked to other documents under the same heading.
Yale Law School, Lillian Goldman Law Library - Avalon Project
http://avalon.law.yale.edu/default.asp
This site offers primary documents important to the field of law, such as agreements, treaties, accords, hearings, etc. Browse through their twentieth century collection, or search through the site for specific keywords that reflect your topic of interest.
http://library.duke.edu/digitalcollections/mma/
Offers health-related printed advertisements from the 1910s to the 1950s, along with explanatory notes. Search or browse through the images.
Central Intelligence Agency - Freedom of Information Act
Search through the titles of documents that have been made available by the Freedom of Information Act. Note that these items are not full-text.