With the Datastore API, structured data like contacts, to-do items, and game state can be synced effortlessly.- Dropbox
I've been working on a web app that lets me tracked all of the TV shows that I'm watching and what episode I am on and I've needed a way to sync it across devices. I was going to build some simple back-endy stuff on Google App Engine, but this just solved my problem. Within about 15 minutes of work I had Dropbox API working and was syncing my app data into Dropbox. SO AWESOME!