One of the more interesting things to happen for me as a result of the air traffic turmoil caused by the volcanic ash clouds from Eyjafjallajökull - other than fretting about my chances of getting to Europe as planned at the start of May (I'm now cautiously optimistic, though hardly because of Qantas's handling of the situation) - was that I discovered the Flightradar24 site, through a link on my colleague Jo Jacobs's blog. What a powerful demonstration of the power of crowdsourced geodata mashups!
If you haven't seen this site yet - go there! It's the kind of user-generated information resource which only a few years ago we couldn't even have dreamt of, but which now relies on no more than the willingness of a few dozen users world-wide to invest a reasonably modest sum in hardware, and of a few hundred more (I'm guessing) to contribute their time to develop the system and curate the data it relies on.
Flightradar24 aims to track - mainly commercial - aircraft, live, as they fly their various routes around the world, and while its coverage to date is mainly focussed on Europe (with a few contributions from the Americas, South Asia, and Australia), for that continent it does exceptionally well. There's no unauthorised use of data involved here - in the first place, the site simply relies on commercially available devices that receive the transponder information (flight number, aircraft type, height, speed, position, etc.) which is constantly transmitted by commercial airliners - and a few others, such as the planes of the Australian Royal Flying Doctors Service.