The Grid System

http://www.thegridsystem.org/
The grid system is a website which informs people who want to know about grids.

"The grid system is an aid, not a guarantee. It permits a number of possible uses and each designer can look for a solution appropriate to his personal style. But one must learn how to use the grid; it is an art that requires practice. ” Josef Müller-Brockmann

It claims to be 'The ultimate resource in grid systems'.

Even just seeing the website before and after it's grid form in very useful, it has shown me what can be achieved using grids, and how complicated a grid can look, which then creates a simple looking site.