Google released google app engine today.
Sign Up (It says its limited and asks you to sign up. I did, and I just got an invite to start using Google App Engine :)
Here are Scobles videos of the announcement:
(They demo creating an application, plus Python creator Guido speaks)
I haven't had a chance to read the documentation as I was having problems getting to it. I tried signing up for the service, but its limited right now, but you can download the SDK to start getting ready. You need to know Python (which isn't a problem, I do most of my stuff in Python anyways :). The SDK is licensed under the Apache license as you can see above. Their are version for Windows, OS X, and Linux.
The readme in the SDK says you have to have Python installed. Once the SDK is installed you can start the dev_appserver in a command prompt:
Here is the simple guest book sample application running. The guest book looks like a Python WSGI application (at first glance)that gets run on port 8080.
Their are more application you can download from the Application Gallery.
Update: I just got an invite to start creating applications:
I have some more notes from the docs here.