I was put on to dropbox a while ago by Jason. It’s basically something like “My Briefcase” but works, and is available on a number of platforms.
You run the client app on multiple machines. Anything in the specified folder is synced between all those computers, assuming they have internet access. It has conflict resolution and change history. You can share parts of the directory structure publicly if you want. It’s great for putting things like wip demos, small codebases/repos etc on so that you automatically have them at other locations, without fiddling with a thumb drive. The other great feature is that it has a web client that can view most content and fetch via http.

The only reason this wouldn’t be great for home backup is that it uses the internet for all traffic. It’s therefore not suitable to backup say gigabytes of photos, art or video heavy projects, music etc between machines on a LAN or between drives within a machine.

Does anyone know of a super simple free software solution? The main requirement I guess is you can tag directories and have them synced up on a LAN (and perhaps certain subdirectories remotely also for offsite backups of important but small data). The other requirement is that it behaves peer to peer and sort of opportunistically. At home we don’t leave our computers on 24/7  and there is no always on filestore pc or anything either. Extra points if it has clients for other OSes.

It wouldn’t be a big project to write something like this, therefore I can’t help but feel it must already exist
(cygwin, rsync and cron don’t count!).