Skyhook Wireless have created a Google Map of San Francisco to show where people congregate during different times of the day. The map has been created for this Sunday's San Francisco marathon.
By creating s heat-map of where mobile phone users are located (based on the wifi signals within range) Skyhook are able to give a pretty accurate overview of current hotspots in the city. The map includes a slider that allows you to select different times during the last couple of weeks (up to about two hours before the current time).
Skyhook Wireless have another example from this year's Boston Marathon. The example highlights location lookups along the Boston Marathon route on a Google Map. These lookups are displayed alongside a Google Map showing the previous Monday's activity, demonstrating the increase in human activity in the area due to the marathon.