Dropbox - backup files between computers instantly

This is only for Windows/Mac only (Sorry linux users, its in progress):

Dropbox is a freeware application and web service which instantaneously backs up and syncs your files over the internet and to any computer.

Once you install the application, it will create a folder on your hard drive.
Any file you put inside that folder will automatically be synced and monitored for changes, and each time a change is saved, it backs up and syncs the file again. Even better, Dropbox does revision history, so if you accidentally saved a file and wanted to revert to an old version or deleted a file, Dropbox can recover any previous version. See the video above for a full demonstration.

Any file that's synced with Dropbox is available on any computer you sync it to or through the Dropbox web interface. During the beta, Dropbox is limited to 2GB of storage space and syncing. The software and service will remain free of charge, but if you need more than the 2GB, premium accounts will be available. Dropbox is currently Windows and Mac only, though the Dropbox blog promises a Linux client soon