Orzeszek Ratings lets you sync ratings and play counts between Windows Media Player and iTunes on your PC.
SmartBackup is an SSD oriented cloning and folder sync utility for Macs.
CloudBerry Backup for Linux allows you to backup Linux to Amazon S3, Microsoft Azure, Google Cloud or other cloud storage of your choice.
FreeFileSync provides data backup and synchronization features on your Windows, Mac, or Linux computer.
Resilio Sync syncs your files automatically between computers using secure, distributed technology (P2P BitTorrent protocol). Basically a BitTorrent for your info.