Microsoft yesterday announced the general availability of Windows Azure Multi-Factor Authentication. In addtion to a username and password users can now also authenticate via an application on their mobile device, automated voice calls and a text message with a passcode.
Multi-Factor Authentication can be used several scenario’s including On-Premise VPNs, Web Applications, SaaS services and custom applications. Windows Azure Multi-Factor Authentication offers two pricing options: $2 per user per month or $2 for 10 authentications.
See this blog article by Scott Guthrie on how to set up Multi-Factor authentication.