On Windows,
Karen's Replicator is quite good and free.
On Mac you can just use a AppleScript that calls rsync to do a backup. The script can automatically be triggered through an event in iCal. An example of the script I use:
do shell script "rsync -rpEgot --delete --ignore-errors Volumes/Photography/ /Volumes/BK_PHOTOS || echo -n"
I'm not a Unix expert so this may not be optimal instructions ("-rpEgot" part) but it works. "Volumes/Photography/" is the source and "/Volumes/BK_PHOTOS" is the destination.
The nice part about both these solutions is that it will only update what has changed. This is key when choosing a software backup solution. You want it to sync your directories and not just backup. A program that is designed to just backup will overwrite the destination files entirely, even if nothing has changed. As you can imagine, that can consume a considerable amount of time unnecessarily.