Dropbox has rolled out two-factor authentication. Currently, it requires the beta build, but it is pretty easy to setup. You can have it send you text messages or use a few other app-based authenticators (like google's).
While I do not keep anything in Dropbox that could ruin me if it was seen by the wrong person, it would be a bit of a pain to have to restore and put everything back in its proper place.
I turned it on and fits nicely with having two-factor authentication for Google.