Rackspace Cloud Java SDK by Rackspace is an open source multi-cloud toolkit for the Java platform that gives developers the ability to create applications that are portable across clouds while having the option to use cloud-specific features. Some of the services that this SDK supports are Auto Scale, Cloud Block Storage, Cloud DNS, and Cloud Databases.
One of the big debates these days when it comes to cloud computing center around portability and interoperatbilty between providers. That is, if you build an application on Amazon's EC2 or Google's AppEngine or Force.com, or store your data on Box.net or Amazon's S3, how hard is it to port your application or move your data to another cloud provider? If you develop on a given platform, how locked-in, or not, are you? And beyond that, could developers benefit from having standardized APIs to develop to without having to learn a new model and interface each time. As you'd expect, there's no easy answer to this.