These sites provide a nice way to understand the OSCommerce and Zen cart integrations, but more generally are a great testbed to better understand Google Checkout Level 2 integration.
The Demos add 2 links to the standard OSCommerce and Zen Cart distributions: MC Log Viewer and Notification Log Viewer. These are Ajax Log Viewers that allow you to look at the callback logs for Merchant Calculations and Notifications, very much like what a tail -f on the log files would provide you on a Unix machine.
In order to experiment with API callbacks, first open the Log Viewer window. These demo sites target the Google Checkout Sandbox, so you can easily create fake orders that will trigger Merchant Calculations or Notifications.
The administrative user interface for both sites is exposed to the public, allowing you to study the lifecycle of a Checkout order, and the database for these sites is refreshed from scratch every hour.