JackRabbit Viewer
Apache JackRabbit is an open source implementation of the JCR specification. Essentially, it's an XML based data store, as opposed to using a relational database model.
To simplify developing applications on top of JackRabbit databases (because that's what I was doing when I wrote this ;-) ), I've written an application that enables you to browse through a JackRabbit JCR database, and also to run XPath queries against it.
Implementation Details
JackRabbit Viewer is built using a number of technologies
The application is free to download and use. You can download it here.