The following resources were shared by the U.S. Army at the following link: http://www.army.mil/d-day/resources.html. They are great for teachers as they prepare lessons and units on D-Day.
The National D-Day Memorial: http://www.dday.org/
The National D-Day Museum: http://nationalww2museum.org/index.html
The United States National Archives and Records Administration: http://www.archives.gov/