We just released our Duo Drupal module yesterday!
For those of you that haven't used our two-factor authentication before, Duo enables convenient and secure two-factor authentication for web sites, VPNs, Unix servers, WordPress blogs, and now: Drupal! Duo's Drupal module adds a second line of defense to your Drupal login credentials – your mobile phone! You and your users will be able to log in to your Drupal-powered site with ease and with confidence that their account is secure.
Check out our project page on drupal.org, our source code on GitHub, and our Duo Drupal documentation that will get you up and running with two-factor auth on your Drupal-powered site in under 10 minutes.