Foursquare
Foursquare is a service that helps you keep track of the places that you visit. Foursquare is available on many devices, such as iPhone, Blackberry, and Android. Every time you visit a place, you "check in" to the place to earn points and badges. Over time, you will acquire a history of places you visit as well as badges. You can share your history with your friends. People have created visualizations of Foursquare check-ins, e.g. WeePlaces and 4sqMaps.