I can't speak for all the libraries linked above, but I've used fake-s3 with a rails app and s3rver with a node app. They both worked great for development and

I can't speak for all the libraries linked above, but I've used fake-s3 with a rails app and s3rver with a node app. They both worked great for development and testing. I'd definitely use em again. I think it's worth noting that some of these libraries have different intended goals. For example: Riak CS is intended as an alternative that you can use in production, while s3rver is meant for testing.