I recently found Balsamiq, a $79 program that beats the pants off more sophisticated (and expensive) systems like Protoshare for doing website/iPhone mockups.

The designer, Peldi, has an unwavering eye towards simplicity and ease of initial use.  Over time you discover powerful features, like image importing, that are a delight to behold.

At left is a quick mockup I literally made in 3 minutes using Balsamiq.  Watch the video above to see how I did it.