This extension can change(fake) the geo location as you want
This extension overrides the getCurrentPosition() method of Geolocation API in HTML5
Hence current position follows the setting you save in popup menu.
some sites seem not to work correctly as user mentioned in review.
I think its due to the timing the override the current position.
If you have a trouble, please notify the web page where this doesn't work.
FYI, try this site http://html5demos.com/geo