JCR specifies an API for application developers (and application frameworks) to use for interaction with modern content repositories that provide content services such as searching, versioning, transactions, etc.
Apache Sling - a web framework for building applications on top of Apache Jackrabbit
Hippo CMS - an Open Source content management system based on Apache Jackrabbit
Jahia - Open Source ECM based on Apache Jackrabbit
Magnolia (CMS) - an Open Source content management system based on Apache Jackrabbit
OpenKM - Open Source KM based on Apache Jackrabbit
Sakai Project - Open Source Collaboration and Learning Environment based on Apache Sling and Apache Jackrabbit
Adobe Experience Manager - experience management system based on Apache Jackrabbit Oak, successor of the Day CQ content management system acquired by Adobe in 2010