#MyFavoriteThings Scünci silicone "gel evolution" hair elastics. I actually don't use them for my hair, though, unless I don't have my usual hair elastics on hand, because I have very fine hair (as in, each strand has a low cross-sectional area), and when you use these as hair elastics, they have a tendency to hold so strongly that they damage and break very fine hair.
However, as general purpose #elastics, these things cannot be beat. Imagine a strong rubber band that never gets brittle or wears out.
I have used these to bundle cables/cords/straps and camping gear (like eating utensils and chopsticks, or tent pegs, etc) as button extenders for skirts when I'm feeling fat, as button retainers for the sleeves of my denim jacket when I turn them back, as shock relievers, as impromptu loops to hang or tie things up, and I'm sure there are uses I'm forgetting. The simple use of a lark's head/cow hitch/girth hitch opens up a world of applications for these.
And they come in a wide variety of colors, so you can use them for color coding, too, although I mostly use black, because the little goth girl in me will never die.
