Ask HN: What prototyping tool do you suggest for non-technical users?

I'm looking for a prototyping tool for mobile apps that I can use with my two non-technical friends to iterate over design and layout ideas. None of us work in the same timezone and due to hardware/bandwidth limitations for one of them, the traditional pen & paper (+ scanning) approach isn't feasible. So I'm looking for an easy to learn tool that we can all use. What do you suggest? Also, any tips from people with similar experiences would be very much appreciated.