Entropy

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Syncplicity [LifeHacker] is a freeware, Windows only (currently) application which keeps folders and files on your computer synchronized across a network. Example uses include tasks such as

• Code: synchronize your code automatically between your home machine and work laptop without commiting it (to svn etc).
• Automatic backups: synchronize your important data to a backup machine automatically without the need for exporting etc.
• My Documents: sync your various my documents to your work computer and your laptop which you use for travelling

As you can see, the uses of a tool like this go beyond simply sharing documents. I, however, am waiting for DropBox.