According to the Airports Council International, the world's busiest airport by passenger traffic is Hartsfield–Jackson Atlanta International Airport with a whopping 95,462,867 passengers passing through in 2012. It makes sense considering it's the home of Delta, the world's largest airline. So one can only imagine what the flight paths of all those planes must look like, which is why Alexey Papulovskiy scraped data from Plane Finder for a month before charting it all out in a beautifully interactive visualisation tool called Contrailz.
You can type in any airport or city and find a heat map of all the activity here. Red denotes low flying planes, while blue markings are for those a little bit higher up. It's unclear, however, where the distinction between low and high is.
Beijing, Heathrow, Tokyo and O'Hare round out the top five busiest airports, and most are just as colourful as Atlanta but a few others have some interesting patterns.