Good/inspiring uses of automation on Mac OS X
Smart Folders are 'virtual folders'. They're useful for automatically adding or removing items that fit certain criteria.

For example, I have a Smart Folder called 'Wallpapers'. I've made the criteria for entrance to this Smart Folder:


.png filetype
2560x1600 pixels


As such files appear (or are removed) anywhere on my system, so they'll be added to or removed from the Smart Folder.



When I'm done I can just delete the Smart Folder without deleting the files that reside inside it.

NB: The default behaviour is to add Smart Folders to the Sidebar. You can uncheck that option when you create the Smart Folder or you can simply drag the Smart Folder out of the sidebar to anywhere you wish. I've chosen the Desktop.

This link has a good overview of how they work.