According to the wikipedia page, Microsoft Live Sync will shortly stop offering the PC-to-PC sync service. There are lots of apps to sync two PCs on the same LAN, but I want to sync two PCs that are in different cities, across the internet, traversing two different NATs, and that requires some kind of service running in the internet that both connect into.

There is already a few questions about syncing folders and files, but this is not a duplicate because none of them answer this basic question:

Microsoft Live Sync works better than RSYNC, or any of the linked SYNC solutions in any of the "not really duplicates" because it works even when the two PCs have NAT and firewalls between them that forbid direct connectivity, because Windows Live Sync has a free always-on internet server that all the client PCs connect into.

I'm looking for a FREE (no-fees) Microsoft Live Sync work-alike PC-to-PC sync solution that works between PCs and Macs, at least, as well as between PCs, and works behind NAT and firewalls at least as well as Microsoft's solution. (Note that Microsoft's solution makes only outbound socket calls to a microsoft server, so this solution must necessarily include a server-hub component that is hosted publically on a free site and which does not require that I set up and manage and pay for my own public internet hosting site)

Hint: None of the answers in the linked duplicate are equivalent (PureSync,FreeFileSync,BestSync 2010,SyncButler,Comodo BackUp,QuickShadow,Gbridge) in that none of them work for the PC to Mac situation, where firewalls and nats prevent direct connection, or else they require money to be paid.

When Microsoft Live Sync / Live Mesh finally kills direct PC-to-PC mode, the limitation will be that you will have to pay for more than 25 GB of cloud service, and you can then only sync PC #1 to PC #2 if you first sync to the cloud, then down to other clients. I can currently sync 100 gb of data from one computer to another, only temporarily "moving the data" through Microsoft's data servers without using up my Skydrive storage quota.