Rare Books Online
Normally, I would not point to an entirely commercial site. I'm making an exception for Octavo Digital Rare Books.
Although the company is selling CDs with digitized rare books and manuscripts, they also offer images of those books online. The text isn't as readable as it would be, presumably, on the purchased product. However, that hardly matters for many of the books offered; they're lovely to look at.
Check out William Blake's Songs of Innocence and Experience (April is National Poetry Month, after all). The print isn't always readable, but the artwork is stunning.