This one, about activity at LAX. After clicking it takes several seconds to load.
A menu of other airports with the same monitors is here.
In keeping with earlier, Kitty Hawk-related comments (in the members’ zone) about the security crackdown in the Wash DC area, it is inconceivable that such monitoring services would exist for DC’s airports. I have become habituated enough to the DC style of operations that I’m surprised but heartened that they exist at all. (I realize that there are for-pay services, I think FlightTracker(?), that do this in a much more detailed way, with specific tail numbers and airline numbers. It’s the open, for-free nature of this site that struck me.)