Wikipedia
jnSynch is a light-weight file synchronization engine written in Java, and released as open source through GPL license.
jnSynch was inspired in rsync and aims to provide an easy way to perform incremental/differential backups that are gentle to computer resources such as CPU usage, memory footprint and network bandwidth usage.
As in PGP (Pretty Good Privacy) toolset, jnSynch can be thought as a Pretty Good Incremental File Sychronization Solution.
It recursively synchronizes a source folder to a target folder. For that, it uses a very simple algorithm of comparing file's "last change date" attribute, between source and target folders, when the attribute is different between source and target, the engine replaces the target file with a fresh copy from the source and updates its "last changed date" attribute to exactly that of the source file.
It was first released to public on September 6, 2008. It is actively hosted in SourceForge.net and code.google.com. It was later scheduled for deletion in code.google.com, so that it is centralized hosted in Sourceforge.Net.