Dropbox saved my ass when my computer completely fried (I tried installing Windows on Boot Camp using a Dell install disc and my computer just refused to turn on). About 6 years' worth of essay drafts, stories, and poetry all got saved and backed up instantly - despite the fact that I'd deleted those stories from Dropbox months ago! I'd moved them to my computer from Dropbox and deleted them, but it saved it in my history.

It's not one of my most-used sites, since I rarely have need for it, but I wish I needed it more, because every time I use it I'm blown away by how easy it makes things.