Can Canadian money (change) be used in the United States?

Generally not. Your Canadian money will definitely not work in US vending machines. Some border towns accept Canadian currency, but the further from the border you get, the less likely it is.

Canadian money in the US is generally not accepted, and will definitely not be taken at the abysmal current rates, unlike US money which you can sometimes use in Canada.

However, in my experience quarters are nearly always interchangeable, I've used US quarters in machines in Canada and American businesses have accepted Canadian quarters in the past.

Devices that take coins in the US are specifically designed to reject all foreign coins, and most will do so quite reliably. If you happen to find one that does not, it would be the equivalent of using a slug rather than a coin in terms of what the potential penalty might be.

In general, foreign coins are much less acceptable than bank notes in most countries (for example, usually banks don't want them and won't supply them), and the US is certainly no exception. There may be a few places in border towns that depend on Canadian tourism that will accept coins and/or bank notes, but it would be stated explicitly. It might even be possible to get a preferential exchange rate, as a promotional deal, but that would be by far the exception.

It depends.

When I was very young (some 55 years ago), living in Lake Hills WA, a small bedroom community just outside Bellevue WA, which in turn is just outside of Seattle WA, Canadian coins and American coins routinely traded one-for-one. I would be very surprised to learn that this was no longer the case, as Seattle is quite close to the Canadian border.

I saw the same thing in St Petersburg FL in 1982. During the high season, the town was close to 40% Canadian tourists.

While there was, in St Pete at that time, about a 20% difference in the value of the coins, according to the exchange rate, the local merchants didn't worry about it. They figured they'd make it back when they passed the coin along to someone else, and it made it easier on the Canadian tourists who were paying the bills.

Anything larger than 25 cents, definitely not.

For pennies, nickels, dimes, and quarters, sometimes.

In Michigan (where I grew up, which is near Canada), almost always. In New York, where I live now, clerks have rejected them.

I've never tried to use more than one or two at once, anywhere.

Vending machines will reject them.

I will add to the existing answers that most Canadian coins, over the last several years (or decades, for some denominations), had been made of (some variety of) a magnetic steel alloy. US vending machines most definitely will not accept any coins made out of a magnetic alloy, and the Canadian government (unlike the US one, ironically) is trying to pull the existing old (including non-magnetic) coins out of circulation (for metal reclamation - many of them are worth more as scrap metal than face value).

So anything you got in recent Canadian change is probably not going to be accepted in a US vending machine, but if you somehow have Canadian coins from the 1980s or earlier (can't recall where exactly the cut-off date is - and it's different for different denominations - but 1980s should be safe enough), they might be accepted, as the size is identical, and the composition is similar.
Of course, if you have Canadian coins denominated 10 cents and up from 1967 or earlier, they are silver, and worth an awful lot more than face value. And, also of course, knowingly passing Canadian currency (either coins or banknotes, though of course the latter is much less likely) as US currency of the same numeric face value (and thus much higher exchange rate) is very illegal fraud either way.

In the eyes of the US Treasury I am sure the answer is absolutely not. Store clerks will sometimes uncaringly or unknowly accept Canadian coins. Any merchant can decide for themselves to take Canadian money in an effort to boost sales just as they could decide to accept Chuck E Cheese tokens if they thought it could boost sales.

There are businesses along the Canadian/US border that are aware of Canadian currency and may accept it at the appropriate exchange rate.

In particular, one company that I worked for on their point of sales software had a not insignificant portion of the business in towns like International Falls and Port Huron be from Canadians. The cashier would select the option for the tender being in Canadian currency and the current exchange rate would be applied to the currency. Outside of those two stores, it is still possible (though neither required nor are the cashiers necessarily trained on that feature of the software).

And so yes. There are places were using Canadian currency (both bills and coins) is perfectly acceptable within the United States. Chances are other companies that have businesses that border the border will have similar policies.

YES. But you should not rely upon it. There is no regulation preventing the acceptance of Canadian currency. If the vendor chooses to accept it, it is accepted. You may struggle to find a store that implements a policy of acceptance but it has happened.

Ask.

Hotels will generally place a notice above the reception desk, and you may be gouged for their inconvenience on the exchange rate.