We ran into the problem of testing our S3 based code locally and actually implemented a small Java server, which emulates the S3 object API. As it might be useful to others, we setup a github repo along with a small website: http://s3ninja.net - all OpenSource under the MIT license.
Being quite simple and minimalistic, this tool is perfect for testing and developement purposes. However, to use in in production, one might want to add some security (altough the AWS hashes are already verified in the API - just the GUI is completely unprotected). Also, it doesn't do any replication or scaling. So this wouldn't be a good choice for large setups.
