Before I launched zsync, I searched around to see if anything similar had already grabbed the name. I decided that I could easily knock the Zaurus file sync tool off of the rankings on Google, and couldn't see anything else too similar.

This paper (and this presentation of it) seems very close to zsync though, and they used the name zsync in fact. Oh well. The approach they describe has some overlap with zsync too: they do the rsync bit on the client side, but with a special server still; and they use compression on deltas (which is different to zsync, which works on already-compressed files).

The decomposable hashes and continuation hashes seem interesting too. I had considered decomposable hashes, but had not come gone into it enough to come up with a good way of using them yet. I don't quite get the continuation hashes yet, but I suspect that both ideas are much easier to do when you have a custom server, than they would be for zsync.