Alternative/compatible datastores for App Engine?

What other "nosql" databases are compatible (more or less) with the Google Appe Engine datastore.

Alternatively, what is a good abstraction layer/solution (f开发者_开发百科ree of charge) that abstracts between multiple database implementations including google app engine and my application for db access.

I'd like to develop an app for GAE but later deploy it on my own server, but I do not want to have to rewrite the db code.

Important Update: The app is developed in Java so ideally the Data Abstraction Layer or any alternatives must be "Java friendly".

The AppScale project has decoupled the database backend so that you can potentially run GAE on a variety of databases. They currently provide support for HBase and Hypertable (ref). AppScale is open source and lets you run your GAE app on EC2 or your own cluster.

Strange as this sounds, but there is a great deal of similarity between AppEngine Datastore and the Lotus Domino: you may need to write an abstraction layer to handle the two APIs, but the capabilities are quite similar. However, you need to be very familar with both models to see the parallels.