I instantly fell in love with Hazel from Noodlesoft the first time I found it back in September '06, and since then it has received a literal landslide of new features that make it even more indispensable to any heavy file trafficker. For those just tuning in, Hazel is fundamentally a file organization utility; it allows you to set up a series of rules that watch files in directories of your choosing, and then it carries out various actions based on your criteria. For example: I download a lot of videos, particularly motion pieces I find from studios all over the web. I also download a lot of DMGs and ZIPs in the name of TUAW, and as you might guess, after just a busy morning of all this watching and downloading, my desktop is about as messy as the typical freshman dormroom.
Enter Hazel, the background file management ninja. For these two scenarios, I have a couple of rules that watch my Desktop for files of type Movie, DMG or ZIP which were added more than 1 day ago. These rules will then move said files into their respective directories (Movies and Software, as one might guess) and apply a blue label to them, just to make sure I know what was moved how.