Beyond Compare - Beyond Compare allows you to compare files and folders.
GoodSync - GoodSync provides highly reliable file backup and synchronization for both individuals and businesses.
WinMerge - WinMerge is an open source differencing and merging tool for Windows.
Syncthing - Syncthing replaces proprietary sync and cloud services with something open, trustworthy and...
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.
rsync - rsync is a file transfer program for Unix systems. rsync uses the "rsync algorithm" which provides a very fast method for bringing remote files into sync.