This book outlines and demonstrate basic layout/grid guidelines and rules through 100 entries.
A useful Mac OS X app that superimposes a customizable grid over any window.
Another excellent article on the Dev Opera site covering baseline grids and how they can be applied to the web.
A Grid-Based-Clock-Screensaver for OS X created by Build.
A simple tool that calculates the overall width of your grid by setting the font size, number of columns, column width and gutter width.
An article on the Dev Opera site on how to implement grids for the web through the use of CSS.