How to keep laptop & desktop files in sync?

corinroyal writes: "I've setup Xubuntu & Lyx on my grandmother's old iBook G3 for my moderately luddite housemate who's writing her novel. I'd like her to be able to write on either her desktop or laptop and have any edits automagically synchronized with the other machine whenever the laptop is connected to our home network. I've looked a bit into options like Rsync, Unison, and the Coda distributed file system. Wondering if anyone here has found a good solution they'd care to describe. To my mind a great solution would have the following:

1. Near real-time, continuous syncing when connected to the network
2. Intelligent options for merging conflicting changes (such as when the same file has been edited on the disconnected laptop and on the desktop)
3. Encrypted network traffic, compatibility with encrypted filesystems like truecrypt
4. Invisible to user except in case of change conflicts

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