I have had this sewing gauge for so many years I can't even remember how old it is! It has been ironed (almost lost it as it warped!!), thrown in the bin with my scraps, lost on numerous occasions! I always panic slightly when I can't find it straight away, after pins and scissors (and sewing machine of course :-)) I think this must be my most used item when I sew! It has inches on one side and millimetres on the other - mentally, I tend to jump between both when I am sewing so I find it really handy. I have never seen another one of these the same. I did find a metal one once but the edges were sharp and it wasn't as nice to use - I also have the terrible habit of putting it between my teeth while I pin and the metal one felt horrid in my mouth, haha - gross I know!
What can't you sew without?