Fabric marking pen

The Frixion pen by Pilot is marketed as an erasable ink ballpoint pen. The "eraser" is actually the hard plastic ball at the bottom of the pen. It supplies the necessary friction to erase the ink and works quite well on paper. 

Available at Office supply stores.
3 pack for $6.99 at Office Max

                             BUT...

it also works very well on fabric as a marking tool. 
It creates a thin, black, very smooth, and easily visible line and is especially useful for marking quilt tops.
If working on a white or very light fabric, you can draw or trace designs with minute detail and no worries of any ink lines remaining visible on the finished piece

                            BECAUSE...

you lightly press the quilt top when finished. The heat from your iron provides the friction to erase the lines and practically no pressure is needed. You don't need to smash the batting or actually "iron". Awesome tool to have when a light pencil line just won't do.

I tested it on several fabrics but use caution on colored fabric. It probably contains a bleaching agent because on SOME of the samples, the black lines disappeared, but left faint white lines in their place. It didn't do this on all colored fabrics - just some. I recommend it for white or light colored backgrounds - especially prints. Test on brightly colored fabric first.

Marked sample

After quilting

After lightly pressing with iron. Look Ma!...no ink lines