This US Army / Marine Corps 281 page Counterinsurgency Field Manual was first released in 2006, and after due deliberation, the military decided to make it open source!
Danger Room has the details.
Part of the rationale for making [the manual] available was the desire to establish an on-going dialog with the American people about the nature of the war we are in. And apparently, there is a long-tail of public interest at work because the manual has been downloaded over 2 million times and is #68 for book sales on Amazon.
Download the manual (PDF, 13.6 MB)