The world's population passed 7 billion at the end of October. If you're anything like us, that number is far too big to fathom. That's where this amazing map comes in. Called Dencity, it visualizes population density using circles of different color and size. It also highlights the 20 largest cities in the world, only one of which is in the United States. The Dencity website shows zoomed-in shots of China (which has six of the top 20 cities) and a handful of other countries, and there's also a print for sale.