RadarVirtuel is a real-time online plane tracking system. It shows currently flying planes mapped to their current position over the land. Each plane is clickable, allowing you to view info about it – flight number, airline, aircraft type, departure, destination, speed, altitude, etc. The information is updated every several seconds, allowing you to follow the planes’ current position in real time. Built on top of Google Maps, it allows you to drag the map, zoom in on a territory, show underlying land as a map, satellite photo or terrain, etc.
Update: Other interesting services for tracking airplanes:
http://www.airnavlive.com – geo mapping of flights, similar to RadarVirtuel
http://data.flight24.com – live flight tracking
http://www.flightstats.com – live flight tracking