I've used it for server backups too, just a simple webserver. Built a script that takes the webserver files, config files and makes a database dump, packages it all into a .tar.gz file on monday mornings, and uploads it to S3 using a "write only into this bucket" access key. In S3 I had it set up so it sends me an email whenever a new file was added, and that anything older than 3 weeks is put into cold storage.

Of course, I lost that script when the server crashed, the one thing I didn't back up properly.