You can currently try out and manipulate over 170 code samples in the Playground, ranging from simple tables, to motion charts based on Google spreadsheets, slideshows based on Media RSS feeds, and various ways of manipulating KML files for Google Earth.
The Playground runs on top of Google's App Engine, which is quickly becoming Google's preferred way of developing and releasing new products.
Ben Lisbakken, a Developer Programs engineer at Google, developed the API Playground as a "20% project." Google also made the source code for the API Playground available under the Apache 2.0 license.
Great for Learning
For students (or anybody who is trying to get his/her head around the basic concepts of using Google's APIs), this could be a great way to get some coding experience with almost immediate feedback and without having to learn the complexities of an IDE.