s3auth.com is a Basic HTTP Auth gateway in front of your private Amazon S3 bucket. Read this blog post for a more detailed explanation: Basic HTTP Auth for S3 Buckets.

Point your test.example.com CNAME to relay.s3auth.com, and register the domain in s3auth.com web panel. You will be able to access bucket's content in a browser with an HTTP basic auth. Your bucket will be accessible using your Amazon IAM credentials and with custom user/password pairs in your .htpasswd file (similar to Apache HTTP Server).

For example, try http://maven.s3auth.com/ (with username s3auth and password s3auth). You will access content of Amazon S3 bucket maven.s3auth.com, which is not readable anonymously otherwise.