I am surprised that no one has mentioned the ruffling foot. I use mine a lot and I prefer it to any of the feet or techniques to gather fabric into ruffles, etc. Because it takes a certain number of tucks per inch-depending on your stitch length, I take a length of fabric that is cut the width of the ruffle desired and feed it through the machine until I have the desired length for the ruffle or flounce. After cutting it, I hem each end if that is needed or just sew the ends into the project as needed. I have enjoyed the cording foot and pin tuck foot. I use #5, the blind hem foot a lot.
I thought about about buying the foot that is used to sew on buttons but did not like the cost--so I figured a way to improvise. I set my stitch length at '0' . I place the fabric and button under the presser foot and carefully determine how wide the zig-zag stitch needs to be. Place a fairly large pin between the holes in the button and hold it there with your finger. My Bernina has that feature of knotting at the beginning of a line of stitching. Now I'm ready to stitch. I hit the knot feature, take as any zig=zag stitches as I want, and hit the knot feature again. Take it out of the machine. Use the tails of the thread to made the button shank by wrapping them around the space created by the needle across the button (between the button and the fabric to which the button is sewed. If your button has 4 holes the process is to do the first two and move down until the needle aligns with the second set of holes. The needle you are using to make the button shank stays in the same place.