In addition to a couple of excellent answers, I'm going to say designer clothing.
I love magazine ads where a model is wearing a plain T-shirt that "only" costs $250, or jeans that cost $500. I lose my mind paying $45 for Levi's on sale; you'd have to be on crack to buy jeans for $500. Not even if I was a billionaire.
I grant you that some stuff that I buy that is on the expensive side is actually worth it. UnderArmour hi-tech fabrics really do work, and they are worth the money to me, not because of the brand name, but because of the function. But I buy generic versions when I can.
Since you had questions about the diamond industry (aka De Beers), you should read this 1982 article. Nothing substantial has changed; the De Beers corp has continued to buy out any new diamond finds that are discovered so that they can maintain tight control of the market.
CONDOMS :-D I think these should be given free of cost from the tax payers money and delivered free with every news paper even to those who don't subscribe to news papers - every government's effort to at least beat STDs where one can
As of fragrances or designer clothes or cars or any such stuff, I LOVE them and think every one of them are priced for what they are worth. If I think they aren't worth it, I won't buy them :-D. it's the brand that sells and brand comes from quality that goes through a lot of research :)
An overpriced product only stays on the market briefly. It's a self-correcting problem. Perfumes are "overpriced" because people pay what sellers are asking. They are by definition, then, priced appropriately.
Starbucks coffee, the building is literally right by my apartment area. I never go there unless my cousin forcibly grabs me by my arm and drags me over there. I just get sad a little in the inside that I'm wasting my money on a drink that I can go buy at Wal-Mart, which is literally right beside of my apartment! and get coffee ice cream and mix it in a blender, I get more out of it and less money to spend :) Even their desserts are overprices at Starbucks, they don't even taste that great for one thing that is like 3 bucks? they better be organic or super healthy if I'm going to pay for a small snack that is 3 dollars.