NYPL Digital Gallery provides free and open access to over 700,000 images digitized from the The New York Public Library's collections, including illuminated manuscripts, historical maps, vintage posters, rare prints, photographs and more. It is a great resource! The site allows you to browse by subject, names, or topic. Some of its collections include:
Illustrated books from the 16th century to the early 20th depicting the animals of the world. Based on the scholarly bibliography of the same title by Miriam Gross, published in Biblion, The Bulletin of The New York Public Library in 1994.
Thousands of art and scientific prints, illustrating medicinal plants, spectacular garden flowers, exotic tropical blooms, trees and ferns. Includes many different printmaking techniques, from woodcuts to stipple engravings to color-printed lithographs.