My Must Have
My favorite all time sewing gadget is my Gingher Snips.  I bought my first one (black and white cord) about 15 years ago.  I can't seem to get rid of it even though the blade has gone dull.  There's a lot of history there!
I love the ease of always having a mini scissor around my neck to cut threads, fabric or rip out a seam.  

