Google App Engine isn't a framework. Google App Engine is platform as a service. The App Engine is a platform for developing applications where the server components are managed for you by Google so that you aren't distracted by the mundane details of configuring a server. It let's you focus on what you do best, building software to solve a business problem.
There are many different frameworks for App Engine. A framework isn't the same as a platform. Take the Java SDK for App Engine for instance. I have a version of the JQuery Form builder, built in PHP, running on App Engine using the Java Querces servlet to interpret the PHP. Another developer I know has used Struts on App Engine, Restlets, and Spring.
Examples of what you can do:
- Host a company website
- Host a live chat application using the ChannelAPI (Comet for App Engine)
- Host a blog using Bloog or other Blog software written in Python
- Many more things
Spring and Struts are MVC frameworks whereas Restlets is a REST framework. The JQuery Form Builder I have running isn't on a framework at all as it was a hackjob solution using Querces to run PHP on App Engine.
In short, there are probably a number of frameworks you can use for Python's SDK for App Engine as well, but App Engine is not a framework.
In short, you can choose to host the application yourself or through a pay-for provider, or you can use Google's next generation model.
Perhaps the most attractive thing for me about Google App Engine is how fast I can get something running, even programming in Java. The App Engine SDK will hot-swap changes by compiling the code after every change. Deployments are also one-click in Eclipse. I can have something in production by just clicking a button.
There is also a lot of scalability on platform as a service models, which can scale up or down depending on the volume on your application.
Lastly, it's free for low usage.