Gridzzly is a beautifully simple tool for anyone who uses grid, graph or ruled paper. The webapp lets you fine-tune the pattern you need and then easily print it yourself.
Just choose the shape (dots, lines, squares, or even hexagons), move the slider to adjust the spacing, and print away. The ruler at the top helps with sizing.
The site was designed by Rostislav Blaha and couldn't be easier to use.