Another excellent online resource: Pragmatism Cybrary.
The Library of Congress site for The Making of America in Books and Periodicals, as well as the Cornell University and University of Michigan versions. All three institutions have contibuted to the project, but do they all need their own interfaces and search mechanisms? Apparently they don't overlap. I'm not sure.
And a couple sites allowing for comparison of texts in the original Greek and as translated into English: The Little Sailing and Ancient Greek Online.