Can't Do Without It
For me, my "can't do without it" product is a Transfer Sling - We use it as a leg-lifter. It's not one of the bigger slings, it's a padded, 24inches long, 8 inches wide with handles at each end sling. Charlie can't move his legs and due to weak upper body and arm strength he struggles to use the transfer board. The sling allows me to help him move his legs once he begins moving his bottom along the transfer board.
Of course there are other products we can't do without, but the sling ranks right at the top of that list.