TinyCat - The online catalog and integrated library system for tiny libraries, powered by LibraryThing.
Goodreads - See what your friends are reading.
DSpace - DSpace open source software enables open sharing of content that spans organizations, continents...
LibraryThing - A home for your books.
Invenio - Invenio is a free, open-source software to run a digital library or document repository on the web.
BookAuthority - BookAuthority collects the most recommended books on business, technology and science - as featured on CNN, Inc and Forbes