The National Park Service's "Living Classroom"

My family and I just returned from Philadelphia and among other things we dragged the kids to Independence Hall and Valley Force.  That led me to see what they have online and it turns out the National Park Service as an entire site dedicated to teachers.  So if you want to go on a virtual tour of Independence Hall, Appomattax Court House, Brown v. Board of Education historic sites, Central High School (AR), Ft. McHenry, Saratoga Battlefield and so much more.  The best part is that the site links to other ones with even more depth such as The Brown Foundation which is dedicated to preserving the story behind the court case of Brown v. Board of Education.