Hmm...central AC, a dishwasher, and premium cable channels (I used to pay for basic cable but we got rid of that years ago).
I would have said diapers (no kids), but I've bought those for charity drives and occasionally when my niece and nephew were babies.
I honestly can't think of anything else. I can, however, think of a list of things I bought once, wondered what the hell I was thinking, and vowed never to buy again.