Books, although I buy a lot fewer than I used to; I'm running out of storage space for the ones I have. I need to seriously clean out the bookcases and update my list... I have so many that unless something is really new, I can't buy anything without checking first to see if I already have it. I generally only buy books if I expect I'll read them multiple times, usually if an author I like is continuing a series; I like to reread the entire series in sequence when a new book comes out, which is much easier if I own it than trying to get the series in sequence from the library.
I also have a closet full of yarn (and I do mean a closet) - I really need to quit buying yarn for the foreseeable future, but new textures, colors, and materials keep coming out, and I want to experiment with them... it's an addiction, I know, but there are way worse addictions I could have.
Realistically, though, I don't spend very much on either of those - I've bought one book in the past year, and I bought that with a gift card I was given a few months ago. My real luxury, the thing I pay for regularly and don't need, is my monthly cleaning lady. I started using a cleaning lady about 18 months ago; I had knee surgery and couldn't kneel, and my mother had foot surgery around the same time and couldn't come help me, so she offered to pay for a cleaning lady for the first two months when I couldn't do things like scrub the bathtub, and I decided I really liked having someone come in once a month to do the deep cleaning - and needing the have surfaces clear for when she comes in (she cleans around things like mail and clutter) really helps me the rest of the month, too.