If you generally prefer to work with only a few Finder windows at a time, you’ve probably disabled the Finder preference (on the General tab) labeled “Always open folders in a new window.” Set up in this manner, you’re in control of window proliferation in the Finder—new windows appear only when you distinctly ask them to appear. For example, when you double-click a folder in a window, the contents of that folder replace the current window, essentially closing the parent folder as the child folder is opened.
If you want to then go back to the parent folder, there are a few methods you can use. You can, for instance, Command-click (or Control-click in OS X 10.5) on the name of the window in the window’s title bar to see a drop-down menu showing the full path to the current folder. Or you can press Command-Up Arrow to navigate to the parent folder—again, this will happen in the same window, so the parent folder’s content replaces the child folder’s content.
But what if you want to navigate up to the parent folder and leave the child folder open in a separate window? As of 10.5—I’m pretty certain this doesn’t work in 10.4—you can use a new keyboard combination to do just that. Press Command-Control-Up Arrow while viewing any folder, and that folder’s parent will open (and become the frontmost window) while leaving the original window open—just like you’d see if you had your Finder’s preferences set to open folders in a new window.
While I generally prefer to work with only one or two Finder windows, this new shortcut is very useful for those times when I need to see both the parent and child folders at the same time.