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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. To get an introduction to Google Maps, click on the video below.

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, photo sites, and so on.  

Check out these sites and blog postings for more information on using Google Maps in mashups:

Google Maps Mania

Google Maps:  100+ Best Tools and Mashups

100 Things to do with Google Maps Mashups