RioFS is a userspace filesystem for Amazon S3 buckets that runs on Linux and Mac OS X. It supports versioned and non-versioned buckets in all AWS regions. RioFS is intended as a storage backend for legacy daemons that cannot talk natively to S3. It handles buckets with many thousands of keys and highly concurrent access gracefully.
Libdroplet is a C library that implements the S3 protocol and facilitates the writing of tools that interact with S3 services. Libdroplet comes with a set of features which enhances the S3 protocol, including a multi-profile system, full multi-threading, virtual directories with true absolute and relative path support, on-the-fly encryption/decryption and buffered I/O, management of storage pricing, and simplified metadata management.