Ranked in these QuestionsQuestion Ranking
Pro Completely free
Source code releases are provided under GPLv2.
Pro It is quite performant
Pro It supports realtime sync
It can be configured to constantly monitor two folders for changes and sync them instantly when a change is detected.
Pro It lets you program batch scripts
You can program your own jobs for execution as a script.
Pro Portable version available
Pro It supports case sensitive synchronization
For Unix-like systems.
It runs on Windows, Linux and MacOS.
Pro It supports long file paths
It can copy files and folders with more than 260 characters in their paths.
Pro It can copy locked files
It supports Volume Shadow Copy Service, meaning that it can copy files even if they are in use or otherwise locked.
Pro It supports versioning
Versioning is keeping multiple instances of the modifications of your files.
Pro It can sync both local disks and network shares
Pro It supports multiple protocols
It will work with MTP, FTP, SFTP, FTPS, and more.
Con Does not preserve folder timestamps when copying
Con Memory hog
It runs a little slow on computers who don't have much RAM available.
Con Limited built in history
The program only remembers the latest set of folders you synced, so you have to save your syncs or create batch files.
Con A little intimidating for novices
If you never ran a file syncing software, this can be a little tricky to configure as your first one.
Con No backup encryption
Con It doesn't run on older Linux systems
It's dependencies don't allow it to run on older systems.