Ask HN: RSS Feed Sync (Google Reader alternatives)?

OK, so this is another Google Reader alternatives question. I know there have been a few discussions regarding this already. However, they all seem to focus on finding an alternative client to view the feeds. This is not what I'm looking for. I'm quite happy with Liferea (and more recently Canto) on Linux and FeedDeamon on Windows. What I find the invaluable about Google Reader the ability to keep my various clients in sync (and particularly the read status of entries, as that's not something one can replicate by shoving an OPML file into Dropbox-- which is my current plan at the moment).

Does anyone know of an alternative (ideally, free) service that would let me do that?