I don't actually know what this nifty little tool is called, but it is a great help to many sewers so I thought I would share it here. Whenever its spotted at workshops, everyone always wants one! Well, it's cheap at about $6 when I purchased mine and I found it in Spotlight.
It has a magnet on the back and you use it alongside your sewing foot to jut the fabric along evenly at the spacing you desire. This is especially helpful for new sewers.
So you just pop it into position and as you sew, you know that you fabric is allocated the same margin as you feed it under the foot.
Nifty, isn't it? I have many sewing machines and not one came with a tool like this, which really makes me wonder why not?