Griddy is a small JQuery plugin thats creates a simple, customizable grid overlay on top of any element. It can also calculate row heights and column widths automatically based on the number of rows/columns and gutter space. There are few options, including: rows, row height, row gutter, columns, column height, column width, color and opacity.
All of the styling is done inline to avoid linking to an external CSS file or having to use images. The numbers on top are column widths, the numbers running along the left side are row heights. You can also look at 960.gs grid overlay which does the similar thing.