I've used unison in the past and liked it. May not fulfill the "zero knowledge" criteria but IIRC it's fairly simple to set up.

http://www.cis.upenn.edu/~bcpierce/unison/

Unison does fulfill the zero-knowledge criteria, but it's not a daemon that runs constantly, although I might give it a shot, thank you.

You might look at Egnyte or Jungle Disk. We've used Egnyte for a few of our clients and had mixed results with reliability. Luckily they give you a 30 day trial, so you can try it out. Their pricing is also pretty decent.

Thank you, both of those look good. Your comment reminded me of AeroFS, a synchronisation program that's peer to peer. I should give that a shot too, as it doesn't need to be zero-knowledge if nobody other than my computers see my data...

I've been running the AeroFS beta for a couple of weeks and am very happy with it.

Nothing high volume or stressful, but it has done what it said it would quickly and efficiently, and the client even worked behind the moderately aggressive filters we have at work.

I just installed it, apart from the fact that it gave me an "AeroFS Drive" I can't remove (I suspect the older version installed this but the new one deprecated it) and the fact that I can't sync any two arbitrary folders, it looks perfect.

I use Duplicity for remote incremental encrypted backups.

http://duplicity.nongnu.org/

It fulfills the "zero-knowledge" and reliable criteria. It's based on rsync. I'm not sure whether you can use it for synchronisation, though, never tried that.

I use duplicity+S3 as well for backups, it's the best for that. Hmm, I might sync everything to my home NAS and back it up to S3 with duplicity, now that you mentioned it, it might actually be cheaper.

Thanks!

Also any of these cloud backups solutions available, that run on Ubuntu?

i know of Ubuntu One, wondering if there're cheaper/better alternatives.

Oh, yes, I forgot to mention, I run Ubuntu so that's a pretty big prerequisite :)

Any reason you can't use truecrypt/whatever your favourite OS provides on top of Dropbox?

Yes, Dropbox will have to up/download the entire multi-gigabyte file container each time, as far as I know.

I'm pretty sure that Dropbox uses [EDIT: was "something like the rsync algorithm."] something like Tarsnap's algorithm: break a file into data-dependent blocks and upload all changed blocks (optionally with rsync). (For a toy implementation, let everything between two NUL bytes be a "block" - you see how this is a big win for detecting changes and deduplication.) This is supported by e.g. http://serverfault.com/questions/52861/how-does-dropbox-vers....

TrueCrypt and alternatives definitely make sure that writing one unencrypted sector only changes one encrypted sector worth of data - compare their performance with gpg'ing the entire disk on each write, and you'll quickly see why.

Hmm, I will have to look into that, although I think that Dropbox only does per-file diffing... Thanks for the recommendation.

EDIT: Interesting, that makes the TrueCrypt suggestion not only viable, but desirable. Thank you.

A past (small) discussion of this -- mostly just a link to information on the Dropbox forums:

http://news.ycombinator.com/item?id=2043191