Working on a content repository for a client at the moment - have been evaluating JackRabbit and Alfresco as a backend repository for an existing site. Anyway - I was already familiar with Alfresco, but spent a bit of time with JackRabbit today to give a comparison. It's a nice simple implementation of the JCR (JSR-170), but suffers a little from lack of doco when it comes to using it in something other than it's standard configuration.
I needed to test it with Mysql on Tomcat 6 - didn't really have any doco on this - so I added a new page to the wiki. Hopefully this is of use to someone:
http://wiki.apache.org/jackrabbit/JackRabbitOnTomcat6
If you are not into Java and are looking for a way to access JackRabbit, there is WebDAV and also Apache Sling - which provides a RESTful api to JackRabbit. It's in the incubator still - but looks quite nice and has some potential.
