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Google Maps is a map service from Google that allows a user to view basic or custom maps (depending on your location) for a given address or location, get driving directions, see satellite images of an address or location, and even get local business information, such as location and contact information.  Mashups, as defined by Wikipedia, are "web applications that combine data from more than one source into a single integrated tool."  While not exclusive to maps, one of the most common mashup applications include using data from Google Maps in combination with other sources, such as real estate listings.

There are literally thousands of mashups that have been created with Google Maps.