were introduced in Mac OS 8.5, released in October 1998. They have been in the Mac OS since then.
Folder actions, for the uninitiated, are AppleScripts that are attached to a folder (a directory if you like). When something happens to/in that folder the AppleScripts that are attached are run. You could, for example, create an AppleScript that opens every new pdf document added to it but leave the word documents alone. You could create a script that modifies all graphics files added to it, converting them to jpg for example. Or you could open a dialog window listing the number of kilobytes the folder contains every time it's opened.
As AppleScript is able to manipulate a lot of things in the system and also able to script a lot of applications the possibilities are virtually endless.
Now, after 8 years of having had the ability, I'm actually using one. The script is actually very simple and basically moves a file to a certain folder when it is downloaded from the internets. This saves me on average 15 seconds each time I download that file, which is a few times a week.
To say that I'm pleased would be an understatement. This may not be a great timesaver but the task of moving a file every few days to a deeply nested folder is repetitive and boring and I can now skip that entirely as the system does it for me automatically.
I'd love to be able to find other uses for folder actions but unfortunately my life isn't really that organised around files that appear or must be manipulated frequently.
Ah well, at least I finally made use of a system function that has been begging me to use it since 1998.