This is my little family of Singer zig zag attachments. The biggest one - the Automatic ZigZagger, takes cams to make different stitch patterns. It comes with a set of four cams, and three more sets of four are available. (If you want to see all the stitch patterns, LOOK HERE.)
The older versions only make only a zigzag, and the width is adjusted by a screw.
However, those two older version have an edge stitcher which can be attached so that you can stitch on the very edge of the fabric or join a piece of lace right to the edge like this:
And they also have a cording slot so you can zigzag over a cord or thread for gathering. Very useful!
The first oldest one shown (one the far left above) not only works great, but is small enough to pop into my Featherweight case. Actually it isn't any bigger than the standard issue ruffler attachment, and somehow it's dainty size just seems more suited to the FW.