What notion can't you live without?
The one thing that's first to be saved when a fire threatens your sewing room.
For me, it's a seam ripper. I have a terrible tendency to sew things on backwards or askew every now and then, and trying to undo a mistaken seam without one is just a nightmare.
A runner up is my seam ruler. I am terrible at estimating inches, and I find a seam ruler also helps me hold down an edge that I'm pressing. And speaking of pressing, my tailors' hams make everything so much easier...