LibraryThing - A home for your books.
DSpace - DSpace open source software enables open sharing of content that spans organizations, continents...
Delicious Library - Delicious Library 3: Catalog your books, movies, music, software, toys, tools, electronics, & video games. Archive your life.
TinyCat - The online catalog and integrated library system for tiny libraries, powered by LibraryThing.
Goodreads - See what your friends are reading.
Invenio - Invenio is a free, open-source software to run a digital library or document repository on the web.