campsite-checker
The most sought-after campgrounds on recreation.gov open their booking windows at a fixed moment and sell out in seconds. campsite-checker is a Python script that drives Firefox through Selenium to watch for a matching site and grab it the instant one frees up.
You describe what you want in an ini file - a [reservation_#] section per attempt, with the park and site ids pulled from the recreation.gov URLs - then start the script shortly before reservations open. When it finds a match it hands the browser back to you with the 15 minute checkout window already running.
Source & setup instructions available on Github.