I've been getting to know Amazon S3 lately, and there are some great things about it. I think it is one of the long list of unpredicted successes that resulted from the near-ubiquitousness of open source software on the server side. We've been using it for "offsite" backup for drupal sites for a while now. We have a script going which runs by cron daily to do the backups. There are a number of ways to do this. We started using S3fs as a way to mount an S3 bucket in the filesystem, then just copy the files to S3. One of the scripts we've use is here. (We also use rsync.) However, S3fs isn't very actively supported or in development. So we're thinking of moving to use S3cmd, which works really well, and is still under active development.