For those not aware, ADS-B is essentially a transmitter in aircraft that periodically report registration number, altitude, GPS coordinates, and airspeed. This is to supplement the current air traffic control system, and eventually take it over. This data is in real time, and not delayed or censored like the FAA supplied data feeds are.
It is not encrypted, so anyone with an antenna and the proper equipment can get the location information of every equipped aircraft within usually a 50-250mi radius, depending on antenna height, receiver quality, topology, etc. While many US domestic aircraft are not so equipped, this is rapidly changing as I believe all must be by 2022. Any aircraft that ever enters European airspace must be equipped, so that means for practical purposes any International flight can be tracked. This includes most corporate jets which are used to fly anywhere but within the United States.
It's trivial to decode in software, and I run a couple installations with hacked up USB DBS TV dongles. They are not the highest quality (a few enterprising folks sell custom ASIC/boards for doing this better), but are cheap and easy to make work.
It's a fun project to hack on for a weekend, I learned a bit about software defined radio among other things. Even fixed a bug in some code that accidentally calculated the aircraft's location to the wrong hemisphere :) Turns out GPS math is somewhat interesting, in the way they encode the data for the little bandwidth available.
This is also the data that powers flightradar24, and probably some other sites out there. Basically a network of enthusiasts that run antenna installations reporting their data to a central server for visualization.
I brought a small antenna and box on vacation with me to Jamaica a couple years back, and was able to track aircraft a good 300nm+ off the coast from the beach. It was fun watching the tracks route around storms and such.