I’ve been using JungleDisk for a week or so and I already can’t live without it.
Of course I use the OS X version, but they have versions for Windows and Linux also.
JungleDisk looks like a mounted drive on your desktop and you can copy files to it or set up automatic backups. It uses AES encryption for all the files uploaded to S3 so even though your stuff is on the Internet, it’s encrypted. (Yeah, so the NSA can probably crack it, but so what, they already know everything you do anyway.)
JungleDisk also has a web front end so you can access your files on S3 from any computer.
I have Time Machine set to back up every hour, then once a day JungleDisk backs up my home folder to S3.
Highly recommended because local backups don’t do any good when your house burns down.
Now all I need is a JungleDisk iPhone client…