Checkin Mania is a Google Maps mashup to track who is checking in around you using FourSquare, Gowalla, Brightkite and Yelp.
To browse a location you can use the search facility or you can just click on the map. The map will then display all the venues around you and show you who are the top users of that venue with the popular location based social media apps.
One really cool feature of this map is the vast array of different map markers that are used. The markers let you tell at a glance whether the displayed venues are bars, cafes, auto-mobile garages etc. The number of individualised map markers is incredible, to the extent that Big Ben has a Big Ben shaped marker and the Statue of Liberty has a Statue of Liberty marker.