Quicksilver

Quicksilver is a light, fast and free Mac application that gives you the power to control your Mac with keystrokes alone. Quicksilver allows you to find what you need quickly and easily, then act upon these objets as you see fit. All with your fingers on the keyboard. As an example, if you want to find an application hidden in the depths of your file system, simply activate Quicksilver with a keystroke, type a few letters of the application's name, then hit Return or Enter to launch it, or tab and type to find an action such as Quit, Trash, Email To..., Run at Login, Move To..., Get Info, Reveal, the list goes on. Quicksilver indexes and parses only selected parts of the contents of your hard drive in the Catalog, which means it locates and displays the items matching your search quickly. Quicksilver has a modular structure which means features for interaction with a whole host of of other applications (iTunes, 1Password, Safari, Microsoft Office) can be done after installing the relevant plugin. Versions compatible with older OSes are available at http://qsapp.com/download.php.