Microsoft Security Development Lifecycle (SDL) is the software security assurance process that introduces security and privacy early and throughout the development process.
SDL Threat Modeling Tool v3 is the core element in the SDL, as it allows software architects to identify and mitigate potential security issues early, when they are relatively easy and cost-effective to resolve.
You can download the SDL Threat Modeling Tool v3, it is still in beta version. 
You may find the demo and tutorial on the tools from here.
To learn more about the SDL Process Guidance, please read this: http://msdn.microsoft.com/en-us/library/84aed186-1d75-4366-8e61-8d258746bopq.aspx
For more information about the SDL, please visit http://www.microsoft.com/sdl
