I use a social network site that attaches location information to posts made on iPhones or MacBooks. When I post from my desktop, no GPS information is available and this field is left blank (there is no provision for user-entered data, unfortunately). Is there some way to spoof GPS on a desktop Mac?
Mountain Lion comes with Localization Services which enables applications and websites to gather and use information regarding your current location. This location is determined using the information of your local Wi-Fi network.
To enable it just go to System Preferences / Security & Privacy / Privacy and check the Enable Location Services checkbox.
Applications to which you have granted the permission to use the Location Services will be shown in this panel. Each application will display, too, if it is turned on or off with a checkbox.
Under the Safari privacy preferences you can find different options for limiting websites access to the localization feature.