Google has announced Google App Engine, which allows you to write applications that run on Google’s infrastructure. I personally would be a bit nervous using the service for anything important—unlike Amazon S3 and EC2, making your own service-compatible alternatives is not feasible, leaving you inextricably tied to Google—but I suppose it could be useful for certain applications that need high scalability, integration with other Google applications, or both.

Note that all 10,000 initial invitations have already been distributed, but you can sign up to be notified when more become available.