This looks promising:

Amazon S3 is a extremely powerful service at the core of Amazon Web Services. However, outside of a production environment, S3 can be challenging to work with. It involves passing keys around, provisioning user accounts, and a reliable network connection — not to mention it costs money.

At Spool, we built Fake S3 to make working with S3 in development and testing environments much easier. Our goal was to make a self contained executable that can mimic the majority of S3 Rest API with few external dependencies.