There's lots of state + national forest land in the Northeast where that's legal. I've done it many times in PA/NY/VT/NH/ME. There are probably some options in the other more developed states too, but I don't usually visit them as much for outdoor recreation.

Outside of the most popular locations for tourists that's pretty much the default.

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If you've had trouble finding this, have you been limiting your search to parks? Parks are usually more heavily restricted in terms of camping.