In addition to being a digital library, it is a linked data experiment with strong accessibility support. It is often cited as an example of linking bib data.
It was created by Aaron Swartz (yes, the Aaron Swartz) and is now a project of the Internet Archive (woot! love the IA).
Linked data info:
http://www.w3.org/2005/Incubator/lld/wiki/Use_Case_Open_Library_Data
Watch it blow up reddit:
http://www.reddit.com/domain/openlibrary.org/