FlightWise.com, with some help from Google, has just released a very nice KML that shows thousands of near real-time flights over the United States.
The data is roughly 15 to 20 minutes behind the current time, due to FAA restrictions. However, this is an excellent tool to get a look at all of the current flights in the US.
By hovering on a plane, you can see the flight number and a path of their journey to that point. Clicking on a plane gives you more information about the flight, including an option to view the full path of the flight as a KML tour. Opening the tour will show your plane with a larger red icon, and it will fly the entire path of its journey to that point.
For more information or to try it for yourself, you can:
• Read about it on the Google Lat Long Blog
• Try it using the Google Earth Plug-in
• Download the KML file for use in Google Earth