Threat Modelling tools?

Hello,

I'm working on a web application for a client that is supposed to be hosted on OS X Server and uses Web Objects so far so good. The client requires a threat model for the application, are there any standard TM tools for this? I know there is a freebie at the Microsoft site but that is a Windows App and it is also taylored a little towards how Windows Apps are written or IIS basewd web apps are written.

I woudl prefer a tool that can relate to OS X ...

thanks

Mike