That's an invisible zipper foot (and some new bobbins). Remember that Alma blouse pattern that I bought? Well, I didn't pay much attention to the details of it at the store, but when I got home I realized that it needed an invisible zipper. Well, crap. I've never installed one before, so I've taken the first step and purchased a foot for it.
In other zipper news, after browsing through the Colette Sewing Handbook, I've come to realize that I've been using the regular zipper foot incorrectly all this time. I can't explain it, exactly, but I seem to be sewing on the wrong side of the foot somehow. Double crap.