There is a really good documentation about how to use Google Maps API, many of the Google examples are with an API Key but you don’t need it. Use the API Key if you want to monitor your apps or if you want to buy additional usage quota (less than 2.5000 loads per day it’s free).

All Maps API applications should load the Maps API using an API key. Using an API key enables you to monitor your application’s Maps API usage, and ensures that Google can contact you about your application if necessary. If your application’s Maps API usage by exceeds the Usage Limits, you must load the Maps API using an API key in order to purchase additional quota.

So, for this basic example, we use Google Maps API without Key to add a draggable Marker in a map that is always centered on our marker position.