Koha
Enterprise-class ILS with modules for acquisitions, circulation, cataloging, label printing, offline circulation for when Internet access is not available, and much more.
Self-hosted Document Management - Integrated Library Systems (ILS) software
An integrated library system is an enterprise resource planning system for a library, used to track items owned, orders made, bills paid, and patrons who have borrowed.
3 apps, ranked by GitHub stars weighted by maintenance health.
Evergreen
Highly-scalable software for libraries that helps library patrons find library materials, and helps libraries manage, catalog, and circulate those materials.
RERO ILS
Large-scale ILS that can be run as a service with consortial features, intended primarily for library networks. Includes most standard modules (circulation, acquisitions, cataloging,...) and a web-based public and professional interface.