960 Grid System

This has been around for a while and is what I use for everything.

Even if you already have a favourite grid, the site is worth a look to see a good showcase of grid based design.

If you download the kit you get templates for Fireworks, Inkscape, Illustrator, OmniGraffle, Photoshop & Visio, as well as Printable sketch paper.

More info here.

Essence

The 960 Grid System is an effort to streamline web development workflow by providing commonly used dimensions, based on a width of 960 pixels. There are two variants: 12 and 16 columns, which can be used separately or in tandem.

The 12 column grid is divided into portions that are 60 pixels wide. The 16 column grid consists of 40 pixel increments. Each column has 10 pixels of margin on the left and right, which create 20 pixel wide gutters between columns.