Wikimedia
Wikipedia: Battle of Antietam
This Wikipedia article on the bloodiest battle in American history gives background information on the battle, the battle itself, and the aftermath.
Curated OER
National Park Service: Antietam National Battlefield
The National Park Service offers a wealth of information on Antietam, the battle, casualties, flags, and weapons. A very worthwhile site.
Civil War Home
Home of the American Civil War: Battle of Antietam
An overview of the Battle of Antietam along with accounts of the battle written by those who fought. Information is from several sources.