Mount S3 (and similar) as a local filesystem using FUSE.

Reported to work very well, although you must compile it yourself as the ZFS and Linux licences prohibit distribution of compiled binaries.

‘A high performance inline data deduplicating filesystem for Linux.’

‘S3QL is a file system that stores all its data online. It supports Amazon S3 as well as arbitrary SFTP servers and effectively provides you with a hard disk of dynamic, infinite capacity that can be accessed from any computer with internet access.’

A write-once system using hashes of data blocks to identify them.

Open-source distributed filesystem. Useful for caching generated images?

FUSE User-space filesystem support comes to OS X.

A log-structured file system in which data is never overwritten.

Cross-platform data recovery tools.

At last! Someone actually implemented a copy-on-write file system.