How can I keep folders synchronized between several machines over my home network?

"You're asking for a lot.

Since internet syncing is out, you can't go Dropbox which is what I would have voted for.

You could take a look at SyncToy, since from your tagging I'm assuming you are on Windows. It is free from Microsoft.

However, this MAY not fulfill your demand of ""not being logged on"". I am not quite sure how to interpret this. SyncToy does not work when you are not logged on on the machine that requires a sync. However, it can pull data form unattended machines (as long as these give access to the protocols that SyncToy uses.

SyncToy has worked for me and for my friends in the past. I am not currently using it, because I switched to a Mac.

Update: from Windows users, I also hear good stories about SyncBack, which offers a free version."

"SyncBack is so good it deserves its own answer :)

I think it meets all your requirements. Almost. I'm not only sure if it fits Isn't limited beyond local available disk space. What do you mean by this?"

"Fairly new on the market (as in it's still in Alpha) is BTSync. It ticks a surprising number of the items on your list and is still under development so more items could well be sorted.

The ability to run BTSync as a service is something that is under development.
Automatically checks the directories for changes
Chunks the file into 4mb sections and only sends across those that have changed (although this algorithm needs some improvement as it doesn't do a differential copy)
Uses LAN transfer whenever possible. Only goes via Internet if it needs to and also supports using relay servers if a direct connection isn't possible.
Doesn't actually support cloud storage so only limited by your disk (although memory usage is high for a large number of files currently).
Copes with handling changes in folders happening offline. Doesn't merge files automatically.

It also supports a bittorrent-style method of distribution, so adding a new node will download from all of the existing nodes, and supports having read-only shares, so that a true-backup style solution can be created."

"I would like to propose a new opensource tool called Syncthing, which is very simple to setup and works over any connection setup (wifi, ethernet, LAN, WAN/Internet) and devices (from computers to smartphones).

I tried it to synchronize my smartphones and my home computer over Wifi, plus my home computer with my work computer over internet, and this all worked nicely.

The only drawback is that both devices you want to synchronize must be turned on during the synchronization, but this is also an advantage because this is due to the fact that there is no middle-man server like Dropbox (no eavesdropping)."

"I use an app called GBridge for a couple things. It creates a VPN and allows local access to PC's even if they're not on the LAN.

I use it for Remove Desktop (well... VNC) from just about anywhere and I have shared folders that auto-sync (on a schedule)

The only downside is that the whole program is a little rough around the edges sometimes..but free apps often are.

I like that it's not server based at all, it just uses google's authentication system not for file transfers.

Data will remain on the LAN if the machines are both local, and use the internet only if needed.

I think the only way GBridge fails to meet your qualifications is that I'm not sure about the login requirements."