Weddings.

Yes, It IS a big day. It's not such a big day that you spend your entire life savings, and have no future.

Get a DJ, get a cake, get a hall, get a photographer.......forget the doves, forget the ice sculptures, forget the wedding planner, forget the genocidial mimes, forget the big limo, keep it small. Do you really need to invite your great aunt, who you've seen 3 times in your life?

You should NOT be spending like $20,000 on a wedding.

The first round of tools for any hobby or DIY project.
If you don't know what you want from a screwdriver, snips, circular saw etc. then there is no point in buying the super primo bells & whistles expensive stuff.
Once you've used a tool and learned what you don't like about it, or what you actually use it for, or how often you actually use it... Then you can make the informed decision to just buy another cheap one, or splash out on something that's actually fun to use.

Buy the 2nd last tool you will ever need.

There are rare occasions where "buy once cry once" apply. But it's rare

Generally, medications. It's pretty rare you have some sort of specific metabolic issue which calls for the branded version; the generic is usually just as good. I have a note in my medical records to NOT give me the branded version of my meds because there's something in the expensive ones that gives me horrific reflux, while the others don't.

My default is to buy the grocery store's house brand unless I can tell the difference.

A 26 ounce can of Morton's iodized salt at my local grocery store costs $2.19. The Food Lion brand costs $0.79. Explain to me why I would pay more than twice the price for name brand salt?

Especially in goods where I know the complete chemical formula of the product like salt and sugar, until I encounter a serious problem with quality or unethical sourcing I'm not going to pay for the brand name.

Tbh about everything. Most of retail is just an industrial scale of the addage "a fool and his money will soon be parted".

Buy second hand, its fine. You probally can figure it out yourself, try to diy. Look at what people are actually doing not the brand of tool they are doing it with. Its a saw, you saw with it, you can get away with sawing a lot of stuff with the same cheap saw.

Soaps are just collections of chemicals, powerwash for example is just dish soap plus water and isopropal alcohol.

You can probably cook it at home. It will probably be better and better for you, because a pound of lard or cup or sugar looks like the red flag it is when you go to cook with it.

Your bed might be better on the floor, then on a frame.

You are probably better off walking or biking then driving.

You probably don't need to watch more shows anyways so why get fleeced to subscriptions. You probably don't need to play games as much so you can pass on that game. You probally don't need to go out for a drink. You probally don't need to go out for a meal. Etc etc

Honestly, I'm a hypocritical ass saying some of this, but its true. The urge to go spend spend spend, isn't a fluke its just successful sociol engineering to separate us fools from our money.

Unpopular opinion but wine.

From my experience majority of people can't distinguish between 5€ wine and 500€ wine. And even if they do, they say it tastes "a bit better", not worth the 495€ difference. Pick one that tastes good to you and don't be ashamed if it's cheap.

Not sure if this counts as cheaping out, but wait a year or so before buying computer games, when the price drops by 50% or more. Some never seem to price drop and others get really cheap right before the sequel comes out.

Over the counter medications. Store brand ibuprofen, allergy meds, cold medicine, etc. Sometimes as much as 1/7th the price, just make sure the active ingredients match amounts and you're set.

New cars. After a car has been owned by one owner, for however short a period of time, it dramatically reduces its price. At least in the UK.

Video games. Unless it's a game I play with friends I typically wait for it to drop in price significantly.

If you're not a contractor, power tools. Buy the harbor freight version first when you need it. If you end up using it enough to break it, then you get a quality one.

Most people are being very specific, but I'd say consumables in general. Rarely is a name brand food or medicine any different than generic. Often they're literally produced in the same factory. Stuff that's meant to last, generally a more expensive product will be made more durable (not always), but this isn't a consideration with consumables. If it's a one-time use or edible, I'm going with the cheapest option 99% of the time.

People are gonna pillory me for this, but flashlights.

First off, you want something that runs off two AAAs, regardless of price. If you can't walk into any gas station, or any grocery store, or what have you, and buy batteries for your flashlight when it dies, it's not gonna matter how bright it was before it died. You also don't want anything brighter than ~200 lumens at the very most, unless you actually need one brighter, for some reason; they drain batteries way faster. You want something thin enough that you're able to clip it inside your pocket and forget it's there. You also want one that has an end switch that toggles between two modes: "full power" and "turned off." If you have one that toggles between low and high settings, you will only use the high setting. If you have one that toggles between low and high settings, and strobe and SoS, you will only use the high setting. Every additional step in between "all the way off" and "all the way on" is just friction you don't need, that will do nothing but piss you off every time you use the damned thing.

The features that make big, fancy flashlights expensive, are anti-features.

Cell phones and plans. Any phone is good enough for regular use these days. And any carrier uses the towers of all the other carriers, it's not like the old days where there was CDMA vs GSM.

T-shirts. Get a 5 pack each of white, black, and another color you like. There, you're set for like a year for $30.

College books.

Headphones. Once you get to the $300 range, the more expensive ones sound different, not necessarily better. I have some electrostatics that have great extension, but the “real” sound is so harsh after a few hours.

Apparentlyv Mr Clean MagicErasers are just melamine sponges which are actually mucho cheapo

Air filters. For car, HVAC, etc. Branded or OEM stuff is usually overpriced.

Wrapping paper and bows/ribbons. THey're just gonna get torn up anyway, no reason to spent a ton of money to make it fancy

Alkaline and rechargeable NiMH batteries. The price per hours of use favors the cheap brands. A top brand might last longer but you are paying a premium for it. Rechargeable NiMH batteries with a good price to battery life ratio are from IKEA.

Toilet paper, trash bags, paper towels. If you go the absolute cheapest, they're arguably defective, but the second cheapest is usually ok.

Saline nasal spray. Just get the generic. It's just freaking salt water.

You should just stop spending money in general if you can. So many responses are like "buy literal garbage like disposable decorations at the dollar store," do not do that. Stop spending money on garbage. Every dollar you spend is damage to the environment and more power for the wealthy, just stop it. Stop spending.
Don't buy entertainment, use archive dot org. Read more. When you like something culture you consumed, give it to somebody else so they don't try to buy it. Pass entertainment around to your friends.
When you have to spend on things like haircuts, repairs, etc, keep the money in your social network. If you don't cook, learn. If you aren't a part of your local grocery distro, join it.

Avoid giving corpos money whenever you can and you'll save lots of money.

Former chef: Knives. My most expensive knife is $80 with a lifetime warrantee. Most are $10-$20. Instead, learn how to use and take care of a knife.

I don't cheap out on things. Rather, when I'm shopping I'm already looking for something that fills the need and is inexpensive. That's the goal. The default stance is to spend as little money as reasonable.