Flight Aware live flight tracking.

FlightAware was the first company to offer free flight tracking services for both private and commercial air traffic in the United States. FlightAware offers live flight data, airport information, weather maps and charts, as well as aviation news to nearly two million users a month. FlightAware also powers operational management and dispatch software, airport FIDS (flight information displays), and provides reporting data to aircraft and airport operators.

Link:
http://flightaware.com

Also, check out Flight View:
http://www.flightview.com/