I saw a short video of an "adventure playground" in North Wales UK, as part of a forthcoming documentary.
I find the whole concept fascinating. On one hand I can imagine the howls of anguish and anger from people worrying about "the children." I can see personal injury lawyers and insurance companies shutting down any sort of US equivalent.
There seems something unbelievably healthy and stimulating about letting kids learn and play in such an unstructured environment. (Note: they are not totally unsupervised, there are adult "play workers" who make sure things don't get out of hand.)
Read more about it here.