If you're one of those people who loves to keep your Mac's desktop nice and tidy, you're going to love Hazel.
It's a simple preferences pane (once installed, you'll find it lurking in System Preferences, not the Applications folder) that acts on rules you create, keeping files in their proper places and tidying up the mess of daily computer life.
Hazel works by applying rules to specific folders. An obvious choice is your desktop, itself a folder in the filesystem. Here, you can create all sorts of rules that will keep the desktop clean. Without any fuss, Hazel shifts downloaded music files to your Music folder and movie files to Movies, where they belong.
It can do much more, though. Thanks to the flexibility of the rules system, you can be quite creative with it. For example, Hazel can automatically change the colour label of a file according to the folder it's in, or according to how you name it or what Spotlight comments you give it. This way, you could colour-code all your project files simply by giving them an appropriate name. You could also save them all directly to the desktop and let Hazel do the filing.
You could also create a folder for storing interesting web page bookmarks, perhaps one that you might resolve to review daily or once a week. Tell Hazel to move anything on the desktop with a webloc file extension to your URLs folder, and all you need do in the future is drag icons from your browser's address bar to the desktop to save them for later. With additional messing around in Automator, you could perhaps establish an automatic web page fetching-and-saving-as-PDF routine as well.
Hazel also includes some Trash management rules, capable of keeping the Trash completely empty, or just preventing it from getting too full (Hazel will trim it once it reaches 1GB, for example).
It's a simple, but delightful and handy little app to have around. You'll wonder how you managed without it.