Tallinn is the most famous free one, I think. In terms of nominal pricing, I was thinking of Grenoble, where it was around $2 USD equivalent when I was there there.

Edit: Another example is Prague, which is around $1.

~$2 fares are common in the US and I would not describe that as "free or nominally priced."

Okay, but do an in-kind comparison: can I take a public tram for ~$2 in similarly sized US cities?

I can’t think of many; the first that comes to mind is Portland’s MAX, which is $2.50, has a smaller network with longer headways, and services a much larger metropolitan area as justification for its existence.

Not sure how the headway and network size compare, but Los Angeles light rail is $1.75. Good deal when you compare the cost of living to most of Europe!

I agree that some European cities subsidize transit somewhat more than most similar-sized US metros.

Tallinn is not (all?) free. The tram is at least €1.50 for 90 mins transfer. Helsinki is €3.10

I live in Helsinki and spent a lot of time last year in Tallinn - the ‘transit is free in Europe’ thing is very much an exception rather than a rule.

Free public travel is only for Tallinn residents (main address registered inside Tallinn city borders).

I think there's also free public travel for tourists with cars if they park their car (in certain places or under certain circumstances).