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.
Goodreads - See what your friends are reading.
Evergreen ILS - Evergreen - Open Source Library Software
Delicious Library - Delicious Library 3: Catalog your books, movies, music, software, toys, tools, electronics, & video games. Archive your life.