Quicksilver is, at a first glance, an application launcher for OS X. It lets you search for and quickly launch your applications using predictive lookups. You invoke the application with a keybinding of your choice, type in the first letters of the program you want to launch (as much as it takes to locate it) and hit return. If you have ever used Spotlight, you know what I mean.
Even though, Quicksilver is much more than a program launcher. It lets you control the active application or look for files in your machine, making it possible to apply many actions to them with few key presses (again, predictive lookups). For example, you'd copy the Application form.txt file stored in your Documents folder straight to the pen drive you just connected by using as few as 7 key presses. It may sound a lot, but if you get used to it, it is a really quick way to work.
But wait! There is much more to it. Quicksilver can be extended by means of plugins. They integrate with other applications such as iTunes, bringing you the ability to "remotely" control them from the keyboard. (For this specific case, I still prefer SizzlingKeys.)
This utility is specially useful when you are working at the keyboard and do not want to move your hands away to reach the mouse. Of course, you first need to get efficient with it to really appreciate its power. I have only been using it for a few days, but it certainly improves productivity in some situations.