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.

The options from Microsoft are Windows Live Foldershare and Live Mesh. Both are under development.

I used to use foldershare and found it quite snappy and reliable.

I currently use Live Mesh. It (currently) offers 5gb of cloud storage, and I use it day to day between 4 computers. It also allows you to remote desktop between computers through firewalls, and sharing folders with other users of the service.

To answer your points though, with Live Mesh:

• You need to be logged on
• Monitors folders
• Only copies changes
• Uses local connections when possible
• Doesn't care if there's no machine on the other end. Can use cloud storage at the same time

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?