Google with Flightwise.com, published a new, dynamically updating KML file of commercial flights over the U.S. "Turn on the layer to see current location and altitude of thousands of flights, in near real-time. Hover over an airplane icon to see airline, flight number, and up-to-date path. Clicking an icon yields an information-rich bubble showing more detailed flight information, including estimating arrival time," explains Google.
And, clicking "Download flight path" link in the balloon downloads a full KML tour of the flight so you can play back and rewind the flight up to its current position and see where it was at any point in time.
Note that due to FAA restrictions and timing of data pushes, the time of the data displayed will lag by an average of 15 or 20 minutes behind the current time.
Download: KML file