I love Pi! It's something that get used frequently by crafters and seamstresses.
Like cone hats- one of the easiest ways to make a cone hat that fits the person wearing it is to make it from a quarter circle. For that, you measure the circumference of your head, multiply that number by 4, divide by pi then divide by 2 for the radius of a quarter circle that will fit your head.
So now you can make your cone with newspaper or on fabric easily, adding a half inch to each side for a seam allowance or overlap.
Adding a brim for witch hat is measuring the circumference of your head and dividing by pi for the inner circle radius, then for the outer circle, add the number of inches you want the brim to be to the radius.
A couple of years ago I posted a PDF template for cutting the waist of circle skirts. You can find that here.
Also check out these printable pi(e) labels.