Useful for displaying pages with a native left-aligned or full screed-width layout on a wide screen.
Add and remove pages at your leisure.
The main purpose I use this for is to center German news sites, some of which are still left-aligned (in contrast to virtually all English-speaking news sites).
I've also found the script useful on text-focused websites which have a flexible (i.e. full screen width) layout like Wikipedia, since it shortens the lines, greatly increasing the readability imho.
The frame width defined in this script is 900px, which has proven to be a good average value and works well for many websites. Once installed, you can adjust this value in the script to perfectly center-align a specific website.