From Michael Lee at ZDNet:
"A few weeks ago, when Dropbox users began reporting that their emails had been leaked to spam lists, Dropbox made some security changes and promised it would bolster its security measures further. The company has now made good on its promise, rolling out the beta version of a two-factor authentication system over the weekend."
I use Dropbox so I suppose that I should try their two-factor authentication system but texting is not available on my cell phone and I don't use any of the other authenticators available for Dropbox.
Dropbox trials two-factor authentication beta | ZDNet