I’ve only just started playing with Dropbox, but it looks very cool.
It’s what iDisk should have been. Software for Windows, Linux and Mac will create a Dropbox folder on your machine. Anything you drop on that folder is efficiently and securely synchronised to all other machines connected to the same account. It keeps past versions of updated files for you. The storage behind the scenes is Amazon’s S3 service. And if you’re using less than 2GB, Dropbox is free.