For about $60 you can have the "poor mans" version of a radar box with ADS-B, ACARS, and off the air ATC reception using three RTL2832U sticks. While it ultimately may not work as well as the $800 boxes, it certainly offers a lot of bang for the buck. This project uses the new RTL1090 Windows decoding software from Jetvision.de and SDR Sharp. It all works pretty well, and is fairly easy to setup. You will need a fairly modest PC around 2.4Mhz with dual cores.
Adding ADS-B, ACARS, and ATC Audio to PlanePlotter Using Multiple RTL2832U Sticks | Ham Radio Science