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...
Readernaut - We are a small community of readers, sharing our adventures and supporting each other as we persue a life of learning.
Invenio - Invenio is a free, open-source software to run a digital library or document repository on the web.
aNobii - aNobii is a social network for listing books, sharing insights, and meeting people.