You can use HTTP calls directly with the Amazon S3 REST API (http://
docs.amazonwebservices.com/AmazonS3/latest/) if you want, but that's
what the AWS::S3 gem is doing for you anyway - it just saves you a lot
of time.
You can use also the Amazon S3 SOAP API. Ruby and Rails will be able
to interact with either API in a fairly straightforward manner, if you
understand SOAP or REST, but neither have S3 support built in.