There are a handful of flight path visualizations out there, but I've always wanted to have a go at my own.
These below videos are built with Processing and, for the moment, assume direct - straight line - flight paths; something that needs to be improved for the next versions.
The first is a visualization of flight departures out of London Heathrow - approximately 530 of them. Their departures and arrivals are not staggered at all, so it is only really an aesthetic visualization at the moment.
The second is of flights departing from across the world - just over 40,000 of them. They change from orange to yellow as they approach their destination.
After a week away, I'm hoping to do a US-centric example; it is astounding how much internal air traffic there is. I'm also looking to add parabolic flight paths and introduce a third dimension (a good example is blprnt's twitter flight analysis).
I'm suspicious that China doesn't have more internal traffic - perhaps something to do with the data source I used; worth looking into.