Google App Engine provides an opportunity to leverage Google’s infrastructure for server side web applications. The platform is built on a Python runtime, includes persistent storage as well as the capability to integrate with existing Google applications (think Google Maps, Gmail…).
The App Engine SDK is an open source project that is hosted on Google code, you can can access the project here. A download of the SDK is available for Mac here.
The App Engine SDK includes a web-server application that provides a means to emulate the App Engine services from within a local development environment. Once deployed, applications can be hosted on the appspot.com domain, or your own domain.
The video below is a good introduction to Google App Engine from a recent CampFire One event:
One of the more intriguing aspects to services like the Google App Engine is that you focus on the application, not the hosting/scaling. Should you build a killer app, your focus still lies on the application itself, not on how to massively scale (which would be a good problem to have). From the App Engine homepage
This is a PREVIEW RELEASE of Google App Engine. For now, account registrations are limited to the first 10,000 developers, and applications are restricted to the free account limits.