The new Google APIs Explorer presents a rich graphical environment that lets the user pick a API to pick a service, pick a version of the API, and explore the different methods available to them.
The Google APIs Explorer includes quick links to the API documentation, and has small help texts for each listed method so one can clearly recognize the purpose of a method without requiring to look it up in parallel.
Best of all, the interface allows you to fill in the parameters (which too have help-texts included with them) and test the actual call and see its response straight from the web interface.
Since some calls, such as getting a list of your Buzz posts, require authentication, the API explorer supports a Prive Access mode in which you can sign in and test how the API calls function when the end-user in authenticated.
Currently only a limited number of a services and methods are supported, however Google hopes to grow the number of supported APIs in the product rapidly over time. All in all it seems like a great new way for budding web application developers to test and experiment with the Google APIs they intend to include in their product.