What kitchen gadget (or any home/garden gadget/leisure item, but try to keep it clean) was going to change your life, but has been gathering dust ever since? Maybe it's the old favourite exercise bike, or a juicer, or pressure washer, or...
Mine was relatively small, but I spent £20 on a terracotta bean sprouting thing, which I used once, then left it on the windowsill for six months until my husband took it straight to the charity shop without asking me. It was like a weight had been lifted.
Conversely, the sodastream, an accidental acquisition, on very special offer for £25, looked certain to fail, but we've used it for fizzy water every day for the last 18 months. I bet plenty of people count that among their white elephants.