ADS-B

ADS-B: Automatic Dependent Surveillance-Broadcast

You can find some excellent descriptions of the system here:

ADS-B Technologies

ADS-B For Dummies

ADS-B on Wikipedia

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, Plotting, and Sharing Software

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

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

Globe-S RTL1090 Edition Designed to plot location data and flight list from RTL1090 output. (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)

Virtual Radar Server Will decode from several ADS-B receivers or can accept networked data feed. Serves a Google Maps-based display of aircraft positions and flight list. (Free) (Windows)

Dump1090 (Free) (Mac/Linux)

Cocoa1090 (Free) (Mac)

Hardware Decoders

Mode-S Beast (PlanePlotter-compatible)

microADSB

Kinetic SBS-3 (PlanePlotter-compatible)

AirNav RadarBox (PlanePlotter-compatible)

PlaneGadget Radar (PlanePlotter-compatible)

Aurora Virtual Radar 3D/SSRx (PlanePlotter-compatible)

miniADSB Kit (Planeplotter-compatible)