Hispanic Digital Library/Biblioteca Digital Hispanica is the website of the National Library of Spain. If you are studying Spanish, here's an opportunity to practice reading it, or if not, you can toggle among several languages (including English) to change headers and instructions on the website.
Many digital collections are available. Some items are simply described, but others--particularly graphic images and audio files--are available for online access. Items from over three hundred years of Spanish history can be found here.