My Singer machine sewing manual says it's an Even Feed Foot. The text says:
"A practical, simple way to accent the lines of a dress or coat is to place one or more rows of stitching along collars, lapels, facing edges, hems, pockets, seams, etc. For a tailor look, use lines of regular straight stitching : sew them with buttonhole twist (or double strands of regular thread) for added emphasis. The Even Feed Foot will help you place stitching evenly and accurately." It also can be used for an elastic stretch stitch. I'll stay tuned to find out what it is from the experts!