I recently start to work a lot with Amazon AWS by utilizing EC2 servers and using S3 as online storage. While you should do your maths using the servers to generously (it quickly becomes expensive if you create instances on the fly and keep them running), S3 usage is a rather innocent matter. Currently 1GB costs you 0.15 U$ per month (more details here), but please dont forget to look at data transfer charges as well (not longer free from November 2010). I use the service mainly for backup purpose, while I still rely on local backups once a month, the continuous backup happens on S3 (which might be safer than my backup harddisk). The data volume I transfer is small enough (not maintaining a filesharing server !) to keep the costs low.
I was looking for a way to transfer my Hudson artifacts to my S3 account and could not find a regular promoted plugin, almost creating a manual script as workaround, until I found a plugin, which you still need by yourself, but works perfectly. This tutorial (based on Ubuntu) is a hands-on walk-through from the scratch. Some steps are rather simple.
In this previous tutorial I described my approach of using artifacts as a backup media.
Update 2010-10-04: There is another updater in github who added functionality to the plugin: here the address git://github.com/d6y/hudson-s3.git