This annotated, well-organized link directory to over 250 digital libraries covers state-level, multi-state, and global resources. It's an excellent starting point for online archival research.
Directory of Archives and Manuscript Repositories in the United States / United States. National Historical Publications and Records Commission. - 2nd ed. Phoenix: Oryx Press, 1988. - National Historical Publications and Records Commission.; Includes index. - Call Number: REF CD3020.D49
Holdings data on 4,560 archival and manuscript repositories in the nation. Despite its age, still useful as a tool for planning research trips. [Choice]
"NARA is an independent Federal agency that helps preserve U.S. history by overseeing the management of all Federal records." NARA is currently undertaking a number of major digitization projects, including the Archival Research Catalog (ARC), an online searchable database containing information about some of NARA's holdings across the country. NARA has already digitized copies of its most popular and significant manuscripts, photographs, sound recordings, maps, drawings and other documents. Approximately 124,000 digital copies are available for researchers to view online through ARC, and the items can be searched by title, subject, or other keywords. Also worth a look is the browseable Access to Archival Databases page. http://www.archives.gov/
Guide to Federal Records in the National Archives of the United States / United States. National Archives and Records Administration, Robert B. Matchette, Jan Shelton Danis, and United States. National Archives and Records Administration. - Washington, D.C.: National Archives and Records Administration, 1995. - compiled by Robert B. Matchette with Anne B. Eales ... [et al.].; 3 v. ; 29 cm; "Supersedes both the 1974 edition and the 1987 reprint. It includes descriptions of federal records in the National Archives of the United States as of September 1, 1994"--Introd.; Rev. ed. of: Guide to the National Archives and Records Administration. 1987.; v. 1. Record groups 1-170 -- v. 2. Record groups 171-515 -- v. 3. Index / compiled by Jan Shelton Danis. - Call Number: REF CD3026
This major reference work provides an introduction to more than 1.7 million cubic feet of textual records, 300,000 rolls of microfilm, 2.3 million maps and charts, 9.2 million aerial photographs, 7.4 million still pictures, 178,000 sound recordings, 123,000 motion picture reels, and 7,000 computer data sets located in National Archives depositories in Washington; College Park, MD; regional branches; presidential libraries; and affiliated archives. It describes NARA's holdings as of September 1, 1994. The Guide is organized into descriptions of nearly 500 record groups, some dating from the prefederal period, that document the executive, legislative, and judicial branches of government. Separately published finding aids noted in some record group descriptions supplement Guide entries. The third volume is a comprehensive index. [Choice]
NUCMC is a free-of-charge cooperative cataloging program operated by the Library of Congress. The purpose of NUCMC is to make available in one database the contents of archival and manuscript collections in research libraries, museums, state archives, and historical societies located throughout North America. From 1959 to 1993, the contents of NUCMC were published in print volumes. Since 1986, bibliographic records created for NUCMC have been made available in electronic format and can now be searched using this database. An important resource for U.S. history. http://www.loc.gov/coll/nucmc/