One of the more unique APIs listed here lately is the FlightAware API. It's a commercial SOAP based API for live and historical flight data. As of the other day, the home page notes "Tracking 5,163 airborne aircraft with 49,337,567 total flights in the database. FlightAware has tracked 38,249 arrivals in the last 24 hours." Here's some details:
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.
What's also interesting is the pricing model:
FlightXML is priced on a volume-basis with a per-transaction fee. There are three classes of transactions based on the complexity of the request. The more queries that a user account makes per month, the less the per-query fee is.
The classes of queries separate the most complex queries for FlightAware to deliver from the more simple queries. For example, requesting a list of aircraft en route to LAX would be a higher class query than requesting the name and location of an airport. Requesting the heading of an aircraft based on two positions would be an even lower class query.
At the lowest end of the volume scale (from 1 to 9,999 queries a month), each query is only $0.012. Therefore, if your application made two queries every 10 minutes, your monthly bill would be $87.84. However, if your application made five queries every 5 minutes, your monthly bill would be $219.60.
Would be good to see more commerical API providers publish their pricing.