Graph paper is handy to have when drawing things and also makes mathematical equations look pretty. Of course, who has graph paper near by? I have plenty of printer paper though, so this free online graph paper site comes in handy.
There are at least forty different types of graph paper to choose from. You have the common grid paper, hexagonal, asymmetric to the interesting circular grid paper. They even have those Cornell Notes papers.
All of this is taken a step further by the PDF generators. You can specify inches or centimeters, document size, borders, grid weight, spacing and of course grid color. I generated one, with all the default settings except with gray lines.