Prologue is the name of the National Archives and Records Administration (NARA) quarterly publication. Although it costs $24 a year to subscribe to the magazine, you can read archived issues back to 1975 for free. The articles are rich in U.S. history, news about the National Archives, and databases to check out.
Here is the link that takes you directly to the Prologue archives page: http://www.archives.gov/publications/prologue/index/#index
But while you are there, go to the NARA home have and look around a bit. There are links galore to get lost in: http://www.archives.gov/