The NY Public Library's Digital Gallery

Officially launched on March 3rd, the NYPL DIgital Gallery is presently offering 275,000 images (stored on a 57-terabyte, a thousand billion bytes of data, network of servers) for public perusal and free personal use ("...individual private study, scholarship and research..."). Most of the contents of the Gallery is in the public domain, and if you can obtain your own reproduction of any image you find here, you can probably use it as you see fit.
The digitized copies on the NYPL website, however, are protected by copyright, and the Library charges a usage fee if an image is used in any "nonprofit or commercial publication, broadcast, web site, exhibition, promotional material, etc" contexts. (It's also possible - for a fee - to order high resolution digital files or hard copy prints of most images through the website.)