The 4.5 million pages digitized so far are available here. They provide information on these and other record groups:
Papers of the Continental Congress (1774-89)
Mathew B. Brady Collection of Civil War Photographs
Southern Claims Commission (1870s)
Name Index to Civil War and Later Pension Files (1861-1900)
Investigative Case Files of the Bureau of Investigation (1908-22)
If your roots are in Early American Jewish families in the north, Sephardic families in the south or among later European immigrants, the information in these files may be valuable.
Previously, there had been no way to easily retrieve names of witnesses and other individuals in Bureau of Investigation records, because the NARA index is only categorized by the name of the person investigated.
According to the press release, the agreement
"will enable researchers and the general public to access millions of newly-digitized images of the National Archives historic records on a subscription basis from the Footnote web site. By February 6, the digitized materials will also be available at no charge in National Archives research rooms in Washington D.C. and regional facilities across the country. After an interval of five years, all images digitized through this agreement will be available at no charge through the National Archives web site."
Archivist of the U.S., Dr. Allen Weinstein, said the agreement will immediately allow access to approximately 4.5 million pages of important documents currently available only in their original format or on microfilm, while Footnote CEO Russell Wilding said they will add millions of original documents and images each month.
Note that Footnote is a subscription site ($99 annual fee) useful for professional genealogists who require 24/7 access. For those readers interested in researching their own families, a less-pricey solution is a one-month all-access subscription for $9.99. You can accomplish a lot in a month! Register as a member of Footnote for free and receive updates as new information is made available.
And, of course, remember that indexes of any kind are only as accurate as the transcriber. Creative spelling is a must when working with any index of historical documents due to spelling mistakes in the original documents, period handwriting and other factors.