I used a machine like that in Sweden or Norway once many years ago, but although it pretended to squish the coin you put into it I am almost completely sure that this particular machine was instead using a different piece of metal that was already flat and softer material than the metal in a real coin.

They do exist here in Germany as well, mostly in tourist destinations. The map does not show any outside the US, though. OpenStreetMap has 662 of them here.

They have this sort of thing on the Canadian side of Niagara Falls, but I think they take nickels now (since pennies haven't been minted since 2012).

Many traditional machines where you put in 2-4 quarters and a real penny have been phased out for credit card readers now that just drop a penny from inside the machine. It does kind of ruin some of the charm.

Destroying currency is crime in Australia, so no, this has never been a thing, here.

I saw one in El Salvador recently (they use US currency, including pennies).

I should have posted the plain URL https://pennypresses.net/ they have an About Us section and more info about the project.

As a person who has never walked by a penny smasher without smashing a penny: Any old copper penny is fine. Pick the cleanest one you have.

Maximum clarity is achieved with pre-1982 pennies. It can be well-worn or have nicely-defined features; that doesn't matter, those features are getting smashed anyway. (Post-1982 pennies are primarily zinc and tend to produce smeary results that have their own way of being interesting, but that's not what you asked about.)

Sometimes, the grime on an old coin gets in the way. You can wash it first. Take it in the bathroom that's always nearby at these touristy places and use soap and water; get it spruced up a bit.

You can even polish it. Coin collectors hate the idea of chemical polishing but they aren't invited to this coin-smashing party anyway. There's lots of different things that can be used to make copper shiny and bright and some are very easy to find while out and about.

Like ketchup, from the burger joint right over there: Just smoosh some ketchup around on a penny that's already fairly clean and the acids in the ketchup will do a decent job of making it bright and pink over the course of just a few minutes. (This works on copper-bottomed cookware, too.)

If it's one of those new-fangled machines that takes credit cards and comes pre-loaded with pennies, then that's fine: Smash some pennies and clean/polish them later.

I'd imagine that pennies minted before 1982 (those that are mostly copper) are best for these machines. The newer pennies (now discontinued R.I.P.) probably aren't great under pressure.

The new machines I’ve used lately, my kid is into it, take credit card and have the penny preloaded

It’s probably more expensive usually $3-5 but also, it’s usually tourist trap pricing. I never have carried a penny on me since probably the 80s when I could actually use it to buy gum

Best to use an old solid copper one, for the USA that's before 1982, for the UK before 1992.