S3sync and s3cmd are ruby interfaces to Amazon S3. S3cmd provides a command line interface to manage S3 buckets. S3sync provides a command line interface to copy data to and from S3.
Bucket Explorer is a user interface (front end) for Amazon S3. It comes in a Windows and a Linux distribution. If you provide "useful" feedback during their beta period, they'll give you a free licence!
Quillen is a backup tool backed by Amazon S3. It's currently (Oct 2007) in Alpha.
This site provides a list of Amazon S3 interfaces, it seemed to be pretty comprehensive as at 27 Oct 2007.
JS3tream will read stdin and write the data to Amazon S3. It can be wsed with tar or gzip for online backup. It can also read from S3 back to stdout. Data is stored in sequentially numbered files, so it could be reconstructed with cat.
JungleDisk is a WebDAV server which stores it's data on Amazon S3 (via a local cache). It's commercial, non-free software for Linux, Mac and Widows and costs $20, lifetime registration.
filicio.us is a backup service based on Amazon S3. You upload your files (via a flash uploader) to filicio.us and they store them on S3. They charge a small premium over S3 prices.