Day CRX is a commercial implementation of the JCR standard and code compatible with the Apache jackRabbit implementation. It comes with a set of tools and better persistance management and clustering.
CRX can be used as a replacement for JackRabbit in a Magnolia implementation and is the basis of the Day CQ5 content management system.
On top of the JCR standard it also defines the backend engine and has a counterpart in the open source world called sling (see sling.apache.org for more information).
Headwire provides a sample package for CRX on the app store implementing a MicroCMS ready to be used in your CRX based applications.
Day CRX now called ADEP
Due to the purchase of Day by Adobe, the Day CRX is product is now called ADEP (Adobe Digital Enterprise Platform) and is the new base for the formerly called Adobe LifeCycle platform.
For more information and a trial download of ADEP visit http://www.adobe.com/devnet/enterprise-platform/trial-downloads.html