My favorite non-standard foot is my rolled hem foot. It has made working on sheer edges a dream. It came to me with a box of old feet from a family member. I have feet in there that I've tried, and they're interesting, but I don't use them. It took me awhile to figure out the foot that automatically gathers. (one set gathering width, so it's not that useful.) This has inspired me to get those feet back out and play some more this winter during my sewing season when the snow keeps me inside.