Unison
Used by 7 peoplecis.upenn.edu ↗

“For that, I use Benjamin Pierce's excellent Unison.”
Marius Eriksen Interview, Software developer (Twitter) · via Uses This 
“To keep all of my files synchronized across the machines I own, I use Unison, a great program that seems to have never gained the traction it deserves.”
John Myles White Interview, Statistics hacker, PhD student · via Uses This 