As I'm redesigning and inventing new UI for FlickrExport 3, I'm finding the Screens tool to be very useful in reminding me
When you have big displays, it's easy to get sloppy with your use of space when laying out UI. xScope's screen tool lets me pop up an overlay at various standard sizes while I'm working in Interface Builder to check that I'm not accidentally making sheets and panels that won't fit on smaller screens.
It's a more useful programmer's tool than it initially seems. Definitely worth checking out.