If you're in Europe and are interested in what's happening in the airspace, take a look at Flightradar24.com
It shows live aircraft traffic in the airspace above Europe. Aircraft equipped with an ADS-B transponder are visible on the map.
You can adjust the size of the planes, and also zoom in to find which planes are overhead!.
Here's a snapshot of what was happening over Heathrow airport (London) as I wrote this posting
There's also more information about the planes and you can chat with others about the radar, too! Might be useful for budding air traffic controllers!!