TinyCat - The online catalog and integrated library system for tiny libraries, powered by LibraryThing.
Koha - Koha is the first free and open source software library automation package (ILS).
DSpace - DSpace open source software enables open sharing of content that spans organizations, continents...
Evergreen ILS - Evergreen - Open Source Library Software
Invenio - Invenio is a free, open-source software to run a digital library or document repository on the web.
VuFind - VuFind is a library resource portal designed and developed for libraries by libraries.