I have an idea for your browser.
Sync localStorage across all the machines I use your browser on.
You'd need the concept of identity, for that I'd strongly recommend just building on Twitter and/or Facebook. I just use Twitter in my systems. Their servers are reliable. And they are not Facebook. :boom:
Then we could talk about an API so other software could access the user's localStorage. Then we'd be getting somewhere interesting, if the installed base of your software took off. Thing is, a feature like this might make it take off.