Ever tried IVAO ?
Very nice experience, with the skies over Europe usually filled with 500 to up to 2000 pilots on the network, and easily 100 or more controllers present on various airports.
An example from the yearly event (called Crowded Skies) to get as much people on the network at the same time : I took a flight from Brussels to Mallorca and had to wait 20 minutes in line for take-off, it was that busy. Then I had complete ATC-coverage down to Mallorca.
Again, that's an exceptionl night, but normal nights are fun as well.
Here's a screenshot from what's going on on IVAO right now, with about 700 people on the network. I've narrowed it down to Europe. The yellow symbols are human-controlled aircraft flying, the red circles are airfields with active/human ATC. The blue areas are human controlled airspaces.