Here’s something to add to my “dive deeper when I have the time” list: Apache CouchDB. It’s a document database accessible via REST, which by itself isn’t terribly unique. What caught my eye was that it was built from the ground-up to be distributed. You can replicate documents across multiple nodes, maintain partial replicas, and sync for offline use. The roadmap has some significant features that need to be implemented before you most people would use it in production, but still, it’s something to keep an eye on.