The UK High Altitude Society (UKHAS) has an excellent predictor that also works quite well in the United States. You can access the predictor directly at http://predict.habhub.org/ and the main site - http://habhub.org/ also has links to other tools and other information as well.

Astra was a flight prediction site that was hosted by the University of South Hampton. What made Astra unique is that it performed a Monte Carlo simulation and would generate a heatmap on the likely zone the spacecraft would land in. Unfortunatly, Astra is no longer online, however, the source code for Astra can be found on GitHub here - https://github.com/sobester/astra_simulator

Amateur Radio is a popular method to track and communicate with your high-altitude balloon. Getting a license in the U.S. (and many other parts of the world) is easy. While usage varies from country to country, in the U.S., the Automatic Packet Reporting System (APRS) is often used to track the balloon. You can also often find fairly inexpensive tracking hardware or build your own.

Image By Cqdx - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=20136993

Satellite is another option some groups have used. While this can be more expensive than other options, you have almost guaranteed coverage anywhere in the world. However, the high operational cost and up front cost may not be for all users.

LoRa is a fairly new technology that uses a new modulation technique to extend the range of wireless communications. Many LoRa modules use ISM bands (so no license is needed) and are inexpensive. If you pair the LoRa with an Arduino or similar microcontroller and GPS, you can build a tracking unit with a range of many miles.