Application launcher and much more.
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.
Quicksilver allows you to find what you need quickly and easily, while keeping your hands on the keyboard. For example, if you want to launch 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. When you don't need Quicksilver, it keeps out of your way, preserving precious screen real estate, and yet you can summon it instantly with a key stroke - just like Mercury the winged messenger. Actually, Roman mythological texts are remarkably silent on whether Mercury could be summoned with a key stroke, but it would have been great if he could. Quicksilver will certainly make you feel as if you have god-like powers.