One of the most popular site recently (within del.icio.us) is the online free download and service of graph paper. Not only it provides 8.5 x 11″ paper (letter size) and 11 x 17″ paper there. You can also custom all those parameters and create your own template.
In full customization page, you can specify PDF Document Size (inches), Minimum Border Width (inches), Grid Line Weight (points), Grid Spacing (lines/inch), and Grid Colour (0-1).
For people who use A4 paper (which what I use), set document size as 8.26″ x 11.69″.
I also set minimum border to 0.25 as I want more grids.
Finally I set the gird color to R:0.7 G:0.7 B:0.7 for light grey color because I only got a B/W laserjet printer.
Sometimes I like to use grid paper for note-making when I do not much writing. I drop down key words and draw graphs or mindmaps.
Grid paper is very useful and it makes your graphs and mindmaps very tidy.