FlightAware API

From their site: Using the FlightXML API, programs can query the FlightAware live flight information and historical datasets. Queries for in-flight aircraft return a set of matching aircraft based on a combination of location, flight or tail number, origin and/or destination airport, aircraft type, and/or a low-to-high range of altitude and/or ground speed, among others. For each matching aircraft, data returned includes the flight or tail number, the aircraft type, origin and destination, time the last position was received, and the longitude, latitude, groundspeed, and altitude of that position. Matching flights' flight tracks can be requested as well. For airports, FlightXML queries can return a list of scheduled flights, flights that have departed, flights that are enroute to the airport, and flights that have arrived at the airport. While supporting full Simple Object Access Protocol (SOAP) and Web Services Description Language (WSDL), FlightXML also supports a simplified query mechanism where the query is coded into a standard HTTP "get" request.