I got an overedge foot with my Kenmore sewing machine. It's supposed to be used when using the overedge or serging stitch, but it can also be used with the zigzag stitch. I've only used it a couple of times, and I'm not sure what advantage it might have over a standard zigzag foot. Here's a link to the one I have:
After looking at my manual again, I realized that mine came with the machine too (Sears exclusive Singer, which is actually a Kenmore). I use it ALL the time in making diapers. I can so easily get a nice zig zag on the edge of fabric with this foot. With a regular zig zag, I'd have stitches that weren't quite at the edge. But maybe that's just because I'm not a great sewer.
Mama to Tornado Boy (6/04), The Brute (11/06), and Mischief (05/09)... expecting in February '15