Quicksilver is a unified, extensible interface for working with applications, contacts, music, and other data.
Anyone using a computer (and the Macintosh is no exception) spends a lot of time locating files and applications, and then carrying out actions on those items. The Finder is a reasonably good tool for getting a visual overview of the contents of your disk, but it isn't ideal for quickly locating and manipulating files, especially if you have a deep and complicated folder structure, or if you prefer keyboard navigation to using the mouse.
