I have started working on the Guvnor project recently. Guvnor manages a SOA repository and sits on top of a JCR implementation. Up until now there really where not too many JCR implementation available. Really JackRabbit was the only viable Open Source gig in town. This has changed now the ModeShape project really has been picking up steam. They recently completed implementing the JCR 2.0 specification. ModeShape has been designed from the ground up with scalability in mind, and it can run in a clustered environment (using JGroups).