Dropbox has launched a datastore API that allows app developers to store data in Dropbox that is not in traditional file formats. This sounds tiny and boring, but for some applications it is exactly the opposite.
For instance, for a long time I have wanted a cross-platform messaging system that is robust in the face of network failure. On top of this, I would build new versions of some personal data tracking applications I have, including time and expense tracking, notes and action lists, plus possibly remote control computer actions. Passing that info via web services is entirely possible, and even easy, but the fault-tolerant network communications, where reliability is far more important than immediacy, would mean writing my own network command buffer, which is a bit of a pain. If Dropbox provides this kind of functionality, they have solved the problem for me.
Mokalus of Borg
PS - I haven't tried using it yet.
PPS - I'll let you know when I do.