Pathsync is a portable professional folder synchronization utility for ad-hoc use, ideal for use on your USB stick. The utility is able to analyze two directories, compare them to the very details, and show the user a list of found differences.
The user can select what actions should occur (which files to overwrite, which to delete, which to ignore), and allow PathSync to synchronize.
PathSync provides nifty statistics during the synchronization, so that the user is never wondering how much time is left, etc.
PathSync is GPL free software, each download package includes the source.