One of my favorite feet for my Pfaff machine is the Pfaff adjustable guide foot.
The bar adjusts so you can stitch an exact amount from the edge of your fabric or another seam. I have used it many times to insure a true 5/8 seam allowance and for sewing even rows of decorative stitches.
There was a discussion about this foot on one of the Yahoo groups. They suggested using this foot for channel stitching. The foot can track 1 inch apart. The next time I quilt, this will be the foot for me. Much easier to attach this foot than the quilting guide that comes with our machines. If you move your needle position you can track up to 1¼ inches apart.
There may be many more uses for this foot or others that I haven’t thought about. Other brands of machines may have a similar foot. Now I must check on a foot for my Viking machine.