FlightRadar24 - Watch air traffic over Europe in real time
This is really cool. FlightRadar24 receives information from several ADS-B receivers in real time and presents this as airplanes on a Google map.
This is what the map looked like back in April when the ash clouds were over the UK. :)
How does it work (More Info)
Flightradar24 shows live airplane traffic from different parts around the world. The technique to receive flight information from aircraft is called ADS-B. That means the Flightradar24.com can only show information about aircraft equipped with ADS-B transponders. Today about 60% of the passenger aircraft and only a small amount of military and private aircraft have an ADS-B transponder. Flightradar24.com has a network of about 100 ADS-B receivers around the world that receives the information from aircraft with ADS-B and sends this information to a server, and then displays this information on a map on Flightradar24.com. Only aircraft within the coverage area of the 100 receivers are visible.