A Safari Extension to show viewport size when resizing browser. You can make it only show the size when you have the Web Inspector enabled and embedded in the browser window (to the right or bottom) by enabling this option in the settings.
It's perfect when you're doing responsive design and use the Web Inspector to resize the viewport (like I do).
Safari 9 for El Capitan now has a built in responsive design mode. Enable "Develop menu" in Preferences and select "Develop" > "Enter Responsive Design Mode". This (more or less) renders this extension obsolete, so I won't be updating this anymore.