The Smithsonian has constructed a nice interactive website on "The Star-Spangled Banner." The song and the flag that inspired it are the primary focus of the site, but there is also information on the War of 1812. The site also includes an audio file of the song as it would have been played in the 19th century.
The National Park Service maintains a first-rate site on Fort McHenry. It includes a solid history of the fort and its origins dating back to the Revolutionary War.