I am currently in the process of testing a program called BTSync or BitTorrent Sync. I know the name "BitTorrent" doesn't have a good legal reputation, but this program uses the distributed architecture of BitTorrent to keep personal files in sync, peer to peer, between private machines. Since Windows Live Mesh shut down, I've been looking for a program exactly like this, and I even realised that BitTorrent would be a good solution except for the difficulties of changing files.
Because of the specific way BTSync works, always overwriting files with their latest versions and requiring machines to be online simultaneously to sync, it's not totally ideal, but it should work for "library" folders like music, pictures and videos. Folders that get added to but aren't often modified.
So far, for me, it seems to be working well. I ran a test on my music folder first, because I still have (almost) all my music on CD, so in case of disaster, I could recover it again. It took a few minutes to index the folder, then indicated, as far as I could tell, that everything was up to date in both places, as I would expect.
Some problems they're working on, as I gather, are the high memory usage and conflict handling, where changes are made in two places at once and you don't want to lose one copy of a file. Still, this has some huge advantages over any other file sync program. One, it can never be shut down by its creators. If they decide not to support it any more, it will continue to work for everyone who is using it. Two, it's free, and will always be so, again because it uses protocols that are out of the control of its creators.
The one huge difficulty it needs to overcome, however, is that the BitTorrent protocol will be blocked by most workplaces. One way around that would be for every workplace to have legitimate uses for BitTorrent programs like BTSync, requiring it to be unblocked by default.
Mokalus of Borg
PS - It's probably not going to happen, though.
PPS - Still, you never know.