1. Method one involves the use of a simple snippet of code in your browser's address bar.
2. Find the location for which you would like to obtain coordinates in Google Maps.
3. Right click on your chosen location, and select "center map here."
4. Open Firebug's console in Firefox then copy and paste the snippet of javascript code below into the address bar, and hit "enter."
javascript:void(prompt('',gApplication.getMap().getCenter()));

5. This will activate a small pop-up window that will show the coordinates for the centered location in decimal format.
    

From: http://gps.about.com/od/gpsproductoverview/ht/google-maps-coordinates.htm