Foursquare just published a beautiful map visualizing a staggering 500 million check-ins from the last three months. The default layer shows check-in locations, and simple toggles on the top left allow you to add context by displaying the data over the MapBox Satellite layer or MapBox Streets layer, which is powered by OpenStreetMap.

Downtown San Francisco

Here are some more screenshots of a few particularly gorgeous looking places I found while browsing the map.

Check-ins in the Midwest

London and the Thames estuary