I've mentioned before how much I like Jungle Disk, and I'm here again today to endorse it some more. They just released a new version that runs as a service. This is good on the Mac because it's one less icon in your dock, and it's good for Windows because it means I can finally run it on my server.
It's only twenty bucks for the software. I've got about 100 gigs of data on S3, and Amazon is billing me less than $20 a month. That's my entire digital life backed up redundantly online. Worth every penny.