Constantinos had a problem that I suspect some other folks have as well. He stores data on flash drives which he carries around between multiple Macs and after losing one of his drives he wanted to work up some kind of automated backup plan. His solution was to work up a method for making an automated backup of the drive every time he plugged it in. Although there were previous approaches to this problem using an Pref Pane called Do Something When, Constantinos wanted a solution that would not depend on having a 3rd party program installed on each of his Macs. Working off of an Apple supplied Folder Action for inspiration, he whipped up a nice solution that uses a Folder Action on the /Volumes folder so that each time he plugs in a drive it runs a backup script that calls rsync to do the actual backing up. (One potential downside of this is that you leave a copy of your data on each machine you plug into).
Check out Constantinos' solution here, but this got me thinking that this would work just fine in the other direction as well. That is, you could set up a Folder Action that would immediately copy over changes in your, say, ~/Documents folder every time you plugged in a flash drive. And of course there's nothing limiting this to flash drives, iPods or external HDs could serve as well. And if you don't have Constantinos' reasons for avoiding it, check out our previous tip using the free Do Something When, which makes this process even easier.