There are a few sites that offer paper (graph, maths, speciality) you can print off (as PDF) for free. Usually with a credit to the site at the bottom of the page.
Konigi offers various storyboard and wireframe PDF’s available in 8.5″ x 11″ (no A4 available). So they are useful when creating a site design mockup or a story.
Print Free Graph Paper offers Cartesian, Engineering, Polar, Isometric, Logarithmic, Hexagonal, Probability and Smith Chart templates available in Letter and A4 sizes.
MathSphere also has paper types the previous site doesn’t.
Incompetech.com has various paper generators for different page sizes (you can create a custom size as well), not just graph paper, but speciality and writing as well (Chinese, Japanese, Fretboard, Cornell etc). You can even define what colour the lines should be.
Useful when you don’t need a full pad, the stationer doesn’t sell the paper you want, or the shops are closed and you need paper before they are open again (you have a deadline or late submission).