Next to Adsense, the most widespread adoption of a Google API on websites and blogs has been Google Maps.
The US National Weather Service has now adopted the API as the default map product when you do a zip code search from their flagship website, www.weather.gov.
The highly localized forecasts, rolled out last year, now include not only an embedded Google Map but also a direct KML link for use with Google Earth:
Thanks to KT for the 'heads up' on this one.