Google's details on maps free usage limits are here and seem to imply the 25,000 map loads per day are per API key:
https://developers.google.com/maps/documentation/javascript/usage
However, if you place a Google Map on a Visualforce page in Salesforce, even the first load of that page will give you the following error:
Google has disabled use of the Maps API for this application. The provided key is not a valid Google API Key, or it is not authorized for the Google Maps Javascript API v3 on this site. If you are the owner of this application, you can learn about obtaining a valid key here: https://developers.google.com/maps/documentation/javascript/tutorial#api_key
The URL of my page is on a Salesforce Developer Edition org:
https://[my_sf_app_name].na15.visual.force.com
Why am I immediately over quota? Is it because map loads are counted not just per API key but also per IP address of the server or per second level domain name (eg force.com) and all my neighbours on Salesforce.com have already shot through 25K map loads for the day before I run my page?