As part of my job as a web developer, I spend an amount of time doing UI prototypes to show the client. It's a pain in the behind but sometimes it has to be done.

I've seen Shuttleworth (and the design team) pump out images like this:

That's made by something called Balsamiq Mockups... Something that balances on top of Adobe Air (yack!) and costs $79.

I've tried it but it kept falling over. I think it had something to do with Air not the app itself. My point is if I'm paying out for something, I want it to be native.