Synchronizing software for use with little USB solid-state disk drive

I got one of those little keychain disk drives that has a flash memory and a USB port on it - just had it a few days but man is it handy.

Somebody must make software aimed at these things, which maybe would keep comparing versions of files on it and on each system you use, and synchronizing files to the most recent version. Or perhaps that would do other useful things I didn’t think of.

Who? What are such products called and what functions have they thought of?

WIN9X has “my briefcase” which will do just that. No need to buy anything