rsync
Used by 8 peoplersync.samba.org ↗

“Then, just for good measure, I clone the entire 1.5 GB Time Capsule to another USB drive via rsync.”
Gary Bernhardt Interview, Software developer, screencaster (Destroy All Software) · via Uses This 

“I also use rsync every day to backup projects to a 3rd location.”
Gus Mueller Interview, Founder, Flying Meat (VoodooPad, Acorn) · via Uses This