The AJAX API Playground was developed as part of the “20% time project” by Google Engineer Ben Lisbakken. According to Ben:
The code for the Playground, which is built on App Engine is open-sourced under the Apache 2.0 license, and Ben has set up a project wiki that includes information on how to add more samples. This is sure to prove a great tool for mashup developers working with the breadth of APIs offered by Google. Great job, Ben!