ADS-B

ADS-B: Automatic Dependent Surveillance-Broadcast

You can find some excellent descriptions of the system here:

There are several professional ADS-B receivers on the market for use by pilots and commercial enterprises, too many to list here. This article will try to cover amateur-level ADS-B reception, decoding, and plotting/viewing solutions.

SDR-RTL Decoding and Sharing Software

ADSB# Decode-only, no plotting capability (Free) (Windows)

RTL1090 Decode-only, no plotting capability (Free) (Windows)

PlanePlotter Plots, shares, feeds other software (Free for 30 days, then Paid) (Windows)

flightradar24 Users are entitled to a Premium account on the flightradar24 tracking site in exchange for sharing ADS-B data. Their software requires a decoder application such as listed above, and does not directly plot data on a GUI or map. ADS-B data is sent to their servers, and is plotted on their web site on a Google-based map. Users in sparse coverage areas may be eligible to receive ADS-B monitoring equipment from flightradar24 in exchange for an agreement to share ADS-B data. (Free) (Windows)

Dump1090 (Free) (Mac/Linux)

Cocoa1090 (Free) (Mac)