I'm now running a homepage (http://www.billkatz.com) on my open-source blog software for App Engine (Bloog). I've included a python shell accessible via your web browser. It's like a limited rails console that lets you mess with your app and datastore in the cloud. (No ssh into App Engine.) Also recently added sharded counters as per some best practices suggested by Google.

I'll report on a more substantial app once it gets some testing.

A good list of apps on App Engine: http://appgallery.appspot.com/