What's a good (& free) file synchronizer?

What app do you guys use to diff/sync files? Only requirement I have is that it's free.

MS Synctoy - you have to run it when you want to sync, doesn't do it automatically, but it works well.

Alternately, you can write your own xcopy script and run it as a scheduled task, such as:

xcopy O:\OMATE32\DATA\*.* "G:\DATA" /e /h /v /d /r /y

to make multiple backups just add more copies of same script in same file like this:
xcopy O:\DATA\*.* "G:\DATA" /e /h /v /d /r /y
xcopy O:\DATA\*.* "H:\DATA" /e /h /v /d /r /y
xcopy O:\DATA\*.* "I:\DATA" /e /h /v /d /r /y

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Instructions:

1. Create a folder named "DATA" on another drive of same computer or another computer on your network - this will be where you backup is stored. (I personally installed a 4GB thumb drive and use that - I used to take it home once a week for backup, but now I just use carbonite.com.)
2. Open Notepad, copy the script into a blank file.
3. Change the "O" to the drive where you keep your main DATA folder. (probably C)
4. Change the "G:\DATA" to the backup location you created in Step 1
5. Save the file as "backup.bat" somewhere safe where it won't be deleted.
6. Open Windows Scheduler, find this file and set it to run once a day at a certain time when computer is on but OM is completely closed out on all computers (we use 10pm) It will search your data folder for changed files (scans, etc) and then update the backup folder.