Strava is an app that uses GPS to enable avid runners and bikers to track their routes and performances. Strava users simply go on a run or a bike ride while carrying a GPS-equipped device, and their data is uploaded to Strava's database.
The widespread use of Strava means that it contains a vast database of running and biking routes; this information has now been depicted in the form of an amazingly detailed heat map showing people's workouts around the world. Strava's dataset includes around 19.7 million runs and 77.7 million rides gathered from 220 billion data points.
You can zoom into the map, pan across it and view popular running and biking routes. The map also contains large swathes of data-less areas, which represent areas where the app isn't used.