Lay out websites easily using 960 Gridder
Once you turn on 960 Gridder, the default grid will be displayed on top of your browser window. To change settings, go to the 960 Gridder sidebar over on the left, and adjust colors, opacity, gutters and spacing to your liking. There's even an option to reverse out the grid, in case you switch from a light to a dark page. 960 Gridder isn't a perfect solution, but it's a free one, and it helps in a pinch.