The JSR-170 specification defines a Java interface for a content repository.

The Apache Jackrabbit http://jackrabbit.apache.org/ project is based on JSR-170.

Also a number of commercial vendors have started their development or have
already released commercial grade repositories that are compliant to JCR v1.0
as specified in JSR-170.

Sakai currently does not have an implementation of JSR-170, but it comes up often in conversation.

See http://jcp.org/aboutJava/communityprocess/final/jsr170/index.html for more information
or to download the complete v1.0 spec.

V2.0 of JCR is under developed in JSR-283 see:
http://jcp.org/en/jsr/detail?id=283

Other opensource implementations include:
http://www.exoplatform.org/company/faces/public/community
http://dev.alfresco.com/
http://www.jeceira.com