I seem to recall reading about an Amazon S3-compatible test server that you could run on your own server for unit tests or whatever. However, I've just exhausted my patience looking for this with both Google and AWS. Does such a thing exist? If not, I think I'll write one.
Note: I'm asking about Amazon S3 (the storage system) rather than Amazon EC2 (cloud computing).
Amazon uses Xen, so you can probably just run your AMI in your own Xen installation. I'd just fire up an instance and run the tests there, though. It doesn't cost much and you should usually be fine with developing locally and infrequently testing it on their system.
Eucalyptus http://eucalyptus.cs.ucsb.edu/
EUCALYPTUS - Elastic Utility Computing Architecture for Linking Your Programs To Useful Systems - is an open-source software infrastructure for implementing "cloud computing" on clusters. The current interface to EUCALYPTUS is compatible with Amazon's EC2 interface, but the infrastructure is designed to support multiple client-side interfaces.
Note that, according to the documentation, Eucalypus includes a reimplementation not only of the EC2 interface but also the S3 storage system. That storage component is called Walrus. (http://open.eucalyptus.com/wiki/EucalyptusUserGuide_v1.5.2)
Are you thinking of Park Place?
FYI, its old home page is offline now.
Park Place has moved to github: http://github.com/technoweenie/parkplace