Jungle Disk is another product (only 20 bucks) that runs on top of the S3 service from Amazon.

http://www.jungledisk.com/

Unlike S3 Backup, it acts as a virtual drive (like a USB drive) that you can drag files to/from.

The interesting part for me, is that they supposedly employ some kind of caching algorithm so that most files (I assume given availabe drive space) are cached locally so you don’t have to go up to the server and download the whole thing when you want it.

Might be worth looking into.