This plugin allows a WordPress Site Admin to enter locations of stores etc into the db. The information is then displayed on the front end 
using the google maps interface. The user can enter a starting location in the search field, the system will then find the closest location from 
it’s starting point and display the path and steps necessary to get to that location. If many locations are available in a similar range range, 
the plugin will request the driving distance between the two closest locations. It help to avoid the pitfall of getting an office across a river 
which may take a lot of driving to get to.
This plugin requires a valid Google Maps API key (free) from Google. If you do not have an API key you may get one here. 
http://code.google.com/apis/maps/signup.html
The actual google maps functionality is based on GoogleMapAPI – A library used for creating google maps. Which can be found here 
http://www.phpinsider.com/php/code/GoogleMapAPI/

GoogleMapper

This plugin allows a Wordpress Site Admin to enter locations of stores etc into the db.  The user can then search for the closest location.

GoogleMapper vs Store Locator for WordPress with Google Maps – LotsOfLocales