What an amazing thread. Mine are:
One of these magnetic racks on the fridge - everyone knows where to find their keys, and we keep return receipts or letters that need action in the tray until they're dealt with.
A pair of scissors in every room. It sounds ludicrous but it is very handy.
All instruction manuals and guarantees are in clear folders, organized by the room the thing is in - e.g. the microwave instructions and oven instructions in the 'kitchen' folder.
I have a charity shop bag on the go at all times, so if I try something on and it doesn't fit or I don't love it, it goes straight in the bag.
Planta app which tells me when to water my houseplants - total game changer.
A clothes ladder in the wardrobe where things that have been worn but don't yet need to be washed hang out.
A giant sharpie for colouring over old address labels if I can't peel the label off.