The Biodiversity Heritage Library

(http://www.archive.org/download/birdsofamericafr01audu/page/n33_w409, "The birds of America" by John James Audubon, 1840-1844)

If you haven't made use of the Biodiversity Heritage Library, you're seriously missing out.

According their website, "the Biodiversity Heritage Library (BHL) is a consortium of natural history and botanical libraries that cooperate to digitize the legacy literature of biodiversity held in their collections and to make that literature available for open access and responsible use as a part of a global “biodiversity commons.”  Which a fancy way of saying they provide knowledge for free, which is awesome (and 'll probably end up doing a post on open access eventually). 

(http://biodiversitylibrary.org/bibliography/15913, "The Natural history of British Butterflies" by James Duncan, 1840)

(http://www.archive.org/download/descriptionofske42owen/page/n183_w543, "Description of the skeleton of an extinct gigantic Sloth..." by Richard Owen, 1842).

You can easily find entire scans of books, which speaks to the way BHL's staff has lovingly digitized their collections. I love this kind of stuff, but I wouldn't have access to it unless BHL existed. The work they're doing is extremely important to help further exposure to and knowledge of the natural sciences.

Plus, it's really pretty to look at.

If you have any interest in natural history, vintage science illustration, or just books in general, this is probably one of the best resources available on the internet.