If you are using Leopard, the Finder has a cool new shortcut that allows you to quickly navigate up the directory tree. When you are in the Finder, just hold down the command key and click on the label for the Finder window. This will show a pop-up window (an example is shown in the window below) that shows the complete directory path to the current file or directory. You can click on any of these entries, and you'll be taken to that directory. If you need to move up several levels in the directory tree, this is a quick way to do that.

The finder after Command + Clicking on the header label