http://www.americaslibrary.gov/aa/all.php (Library of Congress)
I also recommend the databases in Cobb Digital Library as another alternative. (There were other good sites, but I was concerned about some of the advertising and links that might be problematic.)
NEW! Students may now login to Cobb Digital from home using this:
So they would go to
Cobb Digital Library
When asked for a login, they will type in SCH\ and then their student number.