There is a lot of talk about how to exclude, ignore, or delete .DS_Store files on macs. It seems these small files contain data about folders that is used by Finder: http://en.wikipedia.org/wiki/.DS_Store https://wiki.mozilla.org/DS_Store_File_Format
Has this information ever been put to use by a third party program?
Most of the information in those files is accessible through apple events sent to the Finder, and using the apple events is documented. It is things like icon positions, labels and window sizes.
There are applications that will arrange your icons for you. I wrote a screen switcher before Spaces that also switched the icons on your Desktop. It had to indirectly access the .DS_Store to get and set icon positions.
Another related third party use is several applications hide registration and expiration data in ".DS_Store " files.