Whats the best way to create interactive application prototypes?

Question

The question should be interpreted from a general point of view and not targeted solely at web apps or desktop apps.

I have been looking around to find a simple and

Answer 1

There's also Balsamiq. I kind of like it, but usually grow tired of these things quite fast. I end up using either pen&paper or OS X's interface builder, which isn't more difficult to use than all these prototyping tools.