OSS implementation of Google app engine?

After Google pioneered map-reduce the community came out with Hadoop, is there a OSS Google AppEngine project? Or, put another way: What is the best off the shelf python or java cloud software?

Specifically I'm looking for something that I could host on my own and have some sort of auto-scale feature (more frequently used apps would be replicated or something).

Is this a pipe dream? or is there something out there?