Quicksilver
I’ve seen QuickSilver described as a “GUI CLI” — which is to say, a graphical method of interacting with the filesystem whilst using only the keyboard.
Some folks refer to QuickSilver as simply an application launcher. It can be used as that, but the true power lies not in the first pane (showing the application icon), but rather the second and third panes, where you tell apps what to do. As the website says, “launching is a gateway drug.”
If you haven’t used Quicksilver, you haven’t used Mac OS X.