I've long been fascinated by the eXist native XML database, and I think it's just made it into my list of apps that can really help an enterprise out.
Why? Well, I think in the Web 2.0 days there's a clear shift away from aggregated data sources to document-driven workflows. But there's still a need to search, run queries, and limit access to these documents.
Plus eXist belongs to the WebDAV world. That means REST: you can add, update and delete documents from eXist just by issuing the right HTTP command. The icing on the cake is that by making a couple of simple changes to an XML 'collection' (think: collection = database) you can turn it into fully fledged Atom feeds with full APP support. Mmmmmm.