It looks a bit dry when you visit the site but it has many possibilities! This site allows me to use one of their many many free backgrounds and make subtle changes to them. Such as adding a colour to it, then I can choose whether the colour soaks in or sits on top of the design. Lovely subtle effects.
Then I can download it for my own use. How cool is that!

My favourite is the maths copybook background. This is called Old Mathematics.