Turkish towels. Amazing things. Far more absorbent than regular towels, hang them up and they dry in no time at all, fold up tiny so brilliant for the beach or swimming lessons, good to use as a picnic blanket, blanket for the bed in summer when a duvet is too warm, makeshift blind in summer to block the light a bit but let the air in unlike curtains, emergency sarong type thing. They have so many uses. Last for years and the more they get washed, the softer and nicer they get.
Iphone chargers in kitchen, bedroom, one in each car, one at work, plus a battery pack in my bag.
Many pairs of sunglasses, again in both cars, at work and around the house so I can always find a pair (may not be necessary for everyone but I can't see without the bloody things).
Klean Kanteens in various sizes, insulated and regular, I love them.
My mum lives in Ireland and I keep a set of all my toiletries there so I can jump on a plane and bring hand luggage only and not have to bother about liquids/baggage allowance.
Also a spare Aeropress at my mum's so I can have proper coffee without having to waste time and money in coffee shops every morning.
This massive serving spoon which is the best serving spoon ever. No idea why I love it so much but I just really really do. It's nicer to use than anything else I've ever used to heap food into a pasta bowl.
A trolley token keyring so I always have one to hand (unlike pound coins).
Lots and lots of plastic bag clips for re-sealing everything from a loaf of bread to a bag of frozen peas. We have millions of them.