I use "Unison" regularly as a folder synchronization tool. As much as I have searched, I have yet to find something that comes even close to it as a sync tool. However, Unison has its dark side as well. Unison, defying all reason, is not backward compatible with itself, and worse, has no fallback mechanisms built-in.

This means that you need to have exactly the same versions of Unison on both sync endpoints. If you have Unison 2.13 (default in Etch) and Unison 2.27 (default in Leopard and Intrepid), you are out of luck. No synchronization for you!

Upon realizing this sad fact, your next steps should be the classic (a) download source, (b) compile (c) build .deb package, or just cut it short to (e) overwrite the binary.

I found a blog on the Internet where Philipp Heckel finding himself in the same predicament compiled Unison 2.27 and made the binary available. I am attaching the binary in this post, in order to provide a second download point so he doesn't have to bear all the traffic.

He provides full instructions on building .deb packages and also provides a link to the compiled binary. I chose this easier, although not as elegant, method. I copied unison-2.27 to /usr/local/bin and voila!

Thanks Philipp!!