Build a Mockup Application Framework

I am trying to build an application that will help build mockups. So to be clear I am no开发者_开发问答t looking to build an application using mockups but an application that others can use to build mockups. Do you know of any frameworks out there that I can use to build this? This is an application that should be run as a desktop app. I would have liked to use Flash but dont want to take the chance since this might be ported over to the iPad.

Have you looked at Balsamiq Mockups?