The usage of Google Maps is free for non-commercial users. Since 01/2012, commercial users have a current usage limit of 25.000 free requests a day – with additional usage cost of 0.5$/1000 requests. In order to comply with the Google Maps terms of services, commercial users have to register for a free API key. This API key can also be used by non-commercial users in order to monitor their Google Maps API usage.
Please enter your Google Maps API key within the plugin settings / Google tab (available since v2.6):