Published by Chris Coyier
CSS Pivot allows you to apply your own CSS to a site and then share the results (you get a unique URL which opens up that site in a top-bar/iframe dealy). Pretty clever stuff. If you saw a tweak of someone elses that you really liked and wanted to make it persist, you could use this Chrome extension.
I'm posting this as an article instead of a link so we get the comment thread thing going if anyone has any cool ones to share.