Takes a hash snapshot of entire directory structures and can compare any parts of a snapshot to parts of any other snapshots.
Quick, easy, and intuitive interfaces to find differences between directories. View differences between files in multiple ways. Many ways to choose how directories are re-synced together.
Includes server to remotely take snapshots and sync files over a network or the internet.
This is kind of like the Unix rsync utility, which I found out about a number of years after writing this. This is much more user friendly though, and more powerful in many ways :-), though it has less options than rsync.
See Directory Difference in Web Browser for a temporary solution.