FlightAware has an awesome and rapidly growing ADS-B program as well. In particular, one of the easiest and cheapest ways to get into ADS-B flight tracking is with PiAware, which lets you convert a Raspberry Pi into an ADS-B receiver:

https://flightaware.com/adsb/piaware/

If you have a spare Android phone that you're not using anymore, you can even turn it into an ADS-B receiver:

https://flightaware.com/adsb/android/