I keep my presser feet in a small plastic box next to my sewing machine and find the ones I most often use are the edge-joining foot, the zipper foot and the regular foot. I have many others - pin tuck, fell foot, ruffler, hem stitch... but I hardly ever use them. Mostly because I don't know how they work or haven't taken the time to experiment with them. Still, I feel like I should buy every specialty foot there is.
I just recently heard about this website that has a video* for every type of Pfaff foot. I just happen to have Pfaff so that's ideal for me, but I think all sewing machine manufacturers have pretty much the same type of feet. Now maybe I'll actually try a few more presser feet from my box. And judging from the list of videos, there are a few feet I don't have. Of course, do I really need those other feet?
*not sure if there's supposed to be sound. There wasn't for me, but my computer might be a bit messed up and the videos might not be playing correctly.