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Microsoft's website has more details.

Mike linked to Synctoy's release and I downloaded it yesterday and started playing.
I did have one peculiarity in that the second time I did a sync after removing the usb disk and plugging it in again it decided to sync some files again (even though nothing would have changed). This has, so far, been the only time it happened but I'm very impressed with the developers as they saw my comment on Mikes blog about this and then emailed me to ask for more information on the problem. Totally unexpected as this is a powertoy, not supported by Microsoft AND my comments are not visible on Mikes homepage.

I really like the product and at the moment have 4 syncs set up.
2 synchronise the download locations of firefox and thunderbird to my usb disk. Therefore when a new extension is available, I download it to one place, normally on the main computer but it could be on a laptop or remote computer if an extension is released whilst i'm using a different pc) and then when I synchronise the usb disk back home, the latest extension is updated back to the home pc. Now I don't have to worry that I have all the files needed to install firefox (or thunderbird) at a friendsfree client's house.
The 3rd synchronisation contains all the gpx's I download for geocaching. These files are retrieved via email within gsak and I can sync these to the usb drive for use on another pc automatically using "C:\Program Files\Microsoft\SyncToy\synctoy.exe -R" after the download withing gsak.
The 4th synchronisation is an entire copy of the usb drive onto disk. This way I have a backup copy in case I loose the disk or it gets corrupted.
I will be adding some more functionality to synchronise the gpx files onto the SD card used in my pocketpc for caching out in the wild and possibly to synchronise mp3 files from my pc to the creative zen (if thats possible - not had chance to try that just yet.)