LibreS3 is a robust Open Source replacement for the Amazon S3 service, implementing (a subset of) the S3 REST API. It is written in a monadic style, currently using Lwt and Ocsigenserver as implementations.
Standard S3 client libraries and tools (for example s3cmd, python-boto, ocaml-aws, etc.) can be used to access it.
It uses Skylable SX as the storage backend, which automatically provides data deduplication and replication.