Microsoft recently added two-factor authentication to its own Outlook.com just a few months back. Now multi-factor authentication is available as a service on Windows Azure.
Multi-Factor Authentication quickly enables an additional layer security for users signing in from around the globe. In addition to a username and password, users may authenticate via: 1) An application on their mobile device. 2) Automated voice call. 3) Text message with a passcode.
Windows Azure Multi-Factor Authentication offers two pricing options: $2 per user per month or $2 for 10 authentications. Visit the pricing page to learn more.
Source: Scott Guthrie