Jungledisk is great. Its easy to use, cheap and runs on Windows, Linux (includes GUI and CLI) and Mac. You can treat is as a network drive or setup automated backups. The protocol is open so you can use tools like S3 Firefox Organizer to retrieve your data outside of the Jungledisk front-end.
Another big plus is that you can run an unlimited number of instances across any number of machines after paying a one time fee of $25.
I'll also chime in for Jungledisk. I use it on 2 machines at home and a few at work (Windows Vista, Server 2003). The cost and security is really nice.
Jungledisk Plus gives you the option to view files online - so it's a backup + online viewing option (since it uses Amazon S3 and is already online).
That looks really neat. I had wondered about using S3 for backups. I'll definitely have to try it out.
It looks like the "Plus" service's block-level access
and transfer resume would pay for itself pretty quickly with a reasonable volume of data ($1/month vs 0.10 or 0.18 per GB up/down), besides being worth it for $1 anyway.
Another big plus is that you can run an unlimited number of instances across any number of machines after paying a one time fee of $25.