First off, note that you will likely never need a key for small-scale development and testing (such as in the API Explorer or from your command line). If you start getting ‘quota exceeded’ messages, please visit our developer portal to get a key. You can also find documentation there. In most instances you will not need an API key.
Note also that at the moment the API is mostly un-throttled, meaning we currently allow a very large amount of traffic with or without keys. The throttling is mostly intended to stop mass downloads and other TOS violations (thousands of requests from the same IP on a single day).