Food. I buy all of our fruit and veges at the farmers market. It's actually cheaper than the supermarkets, but if by chance the prices increased, I wouldn't swap over.
Coffee. I just can't drink nasty cheap coffee, or instant coffee. It's probably one of the most expensive things we buy when you consider how much you get for how much you pay. (around $8 for 250g).
Petrol. I've heard both sides of the E10 argument (there's nothing wrong with E10 vs. premium is just a ripoff vs. E10 is bad for your engine etc etc), but the fact remains that I do notice that my car really does perform better when it has premium fuel in it. So I buy it.
Our Dog. I buy him fresh meat (as in - meat that you would buy for a human, like rump/porterhouse/chuck etc) and I chop it up and mix it with rice and/or pasta and vegetables. I am sure it probably costs more than buying him canned food, but honestly I wouldnt have a clue as I have never even looked at the price of canned dog food, let alone bought it. Our vet tells us he wishes that more dogs came in looking as healthy as our boy, so I believe we are doing something that matters
Internet access. I might look at ways of reducing the monthly cost, but I don't think I would ever consider NOT having it. I just use it too much, for work, information and personal.
I am sure there are many other things I might have a hard time cutting back on, but these are things that come to mind straight away.