FreeFileSync is a free, open source folder comparison and synchronization software for Windows, Linux and Mac OS X. It also works on 64bit operating systems on all platforms. The program can be installed in two versions: as a standard installer on Windows or as a portable application (no registry entries). Powered by a clean, easy, intuitive interface, this tool offers a myriad of features for synchronization purposes.

Features

•Detect moved and renamed files and folders
•Copy locked files (Volume Shadow Copy Service)
•Detect conflicts and propagate deletions
•Binary file comparison
•Full support for Symbolic Links
•Automate sync as a batch job
•Process multiple folder pairs
•Comprehensive and detailed error reporting
•Copy NTFS extended attributes (compressed, encrypted, sparse)
•Copy NTFS security permissions
•Support long path names > 260 characters
•Fail-safe file copy
•Cross-platform: Windows/Linux
•Expand environment variables like %USERPROFILE%
•Access variable drive letters by volume name (USB sticks)
•Native 64-bit support
•Keep versions of deleted/updated files
•Optimal sync sequence prevents disc space bottlenecks
•Full Unicode support
•Highly optimized performance
•Include/exclude files via filter
•Local and portable installation
•Handle daylight saving time changes on FAT/FAT32
•Use macros %time%, %date%, et al. for recurring backups
•Case sensitive synchronization
•Built-in locking serializes multiple jobs running against the same network share

Those are just a couple of its features (not all), there are many others that are improved and added constantly.