Draws a grid on top of a website. This is useful for checking designs that are supposed to follow a grid-based layout.
Right click anywhere on a website, and go to GridFox > Toggle Grid. In the bottom right, you'll see some buttons to help you create, edit and save a grid. You'll also see some arrows which let you browse any existing grids for the site you're looking at.
Want to see it in action? Just try going to nytimes.com, wired.com, or puidokas.com. Once you toggle it on, the appropriate grid will be downloaded and displayed.