For the curious, the rsync algorithm is, at its core, ridiculously small[0] and involves rolling checksums. I just keep wondering why nobody (but Dropbox) bothers to implement it.
That's a really nice resource, thanks for the pointer.
The ideas behind are actually used in multiple places, through the librsync project (https://github.com/librsync/librsync). rdiff-backup and duplicity are 2 known examples, and surely in other projects as well.
[0]: https://github.com/lloeki/rsync/blob/master/rsync.py