A few days ago I bought Hazel, when MacUpdate offered it as their daily promo. After trying it for only a few minutes I was hooked.
Hazel watches any folders you specify and automatically applies rules you create to items in that folder. Rules are defined almost exactly the same way as in Apple Mail, specifying properties to match and what to do with the items it matches. One use is to watch your download folder and automatically move music, pictures, and movies to the appropriate folders.