On the NATCA (National Air Traffic Controllers Association) website is this U.S. map showing every airline flight currently in the air. The information is delayed by 5 minutes. Also, the graphic isn't interactive, so you can see any information about the dots (like which flight it is).
You can also zoom into nine select cities to see the flights in the air and the flight numbers. So the next time you're lying in the grass with your kids (in one of the major cities) looking up at the sky, you could (if you wanted to) figure out where that airplane is going.
See also: Flight Patterns