Freedom of Information Act (FOIA)

A guide intended for researchers who which to make FOIA requests or locate released documents from the federal government and state agencies

Federal Government Documents

US National Archives: Project BLUE BOOK
The National Archives' Project BLUE BOOK collected all records from the US Air Force dealing with UFO sightings. Their claim is that "Project BLUE BOOK has been declassified and the records are available for examination in our research room. The project closed in 1969 and we have no information on sightings after that date."
National Security Agency/Central Security Service: Unidentified Flying Objects
Declassified government documents from the NSA addressing the existence of UFOs.
The FBI Vault
Declassified documents from the US Federal Bureau of Investigation. There's an index on the right side of the page; check out "Unexplained Phenomenon."
FOIA Electronic Reading Room (from the CIA)

The Black Vault
An extensive collection of declassified federal government documents on the paranormal and UFOs. Caveat: These documents are often presented with biased or slanted commentary.