Windows/Mac/Linux: Java-based DirSync Pro provides cross-platform file synchronisation with an easy to use interface, incremental backup and a profile system.
DirSync Pro is a robust and open-source file synchronising tool. Thanks to its Java-based construction it has cross-platform support and a self-contained directory for easy portability.
You can use DirSync to perform bidirectional and unidirectional syncs, making it easy to only update files on one side of the transfer or sync both locations to each other. There are numerous variables you can tweak to control the transfers including only syncing new, larger, or modified files. You can set filters to include or exclude sub-directories, create backups of the synced and changed files, and create configuration files for your most frequently used locations and settings.
The configuration files are handy feature, especially if you're running the application in multiple environments off your USB drive. You can create unique setups and execute them independently making it super easy to create a specific routine for backing up files at work, syncing files at home, or whatever situation you find yourself in. DirSync Pro is open source and works wherever Java does.