Plain yogurt is kind of, well, plain. But drain off the liquid, and you get the same thing as Greek yogurt. (Regular yogurt has a higher whey content than Greek yogurt, which is why it isn’t as thick. It’s also a little saltier and sweeter, because whey contains sodium and sugar.)
Line a sieve with a coffee filter, or even heavy-duty paper towel, place it over a bowl, and pour in your yogurt. Then let it sit in the fridge (though it’ll drain faster at room temperature). The longer the yogurt drains, the more whey it loses, and the thicker the yogurt becomes.
After about half an hour to an hour, you should get something that’s more dessert-topping like (mix in a little honey), to use instead of whipped cream or custard. After several hours, the yogurt becomes a good low-fat sour cream or mayo alternative. After 12 hours or more, the yogurt starts to turn into something similar to cream cheese. The longer you wait, the more spreadable the “cheese” will become.
Of course, you can buy all these products at the store. But making them yourself is cooler.