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Some people prefer it to spotlight, its an application launching search thingy, download it and try, you can always uninstall it after 2 minutes if your'e not happy.
 
Short answer: It's a application (and file) launcher, which lets you open any application or file on your Mac with a few keyboard shortcuts.

Long answer (from Wikipedia): Quicksilver is a free utility made by Blacktree Software for Mac OS X. It allows users to use the keyboard to rapidly perform commands such as launching applications, manipulating files and data, running scripts or sending e-mail. It is similar to the Mac OS X applications LaunchBar and Butler, though it differs from these in terms of the interaction paradigm it employs. Although Quicksilver is a highly flexible and complex application, it is centered around a very simple three-panel interface (called the "command window"): the user easily performs complex tasks using simple key-combinations in a set (and configurable) order. [...]
 
i use it to keep my dock clean, i hate it if there's a thousand icons down there. so now, instead of going into finder going through a lot of folders to find the application or document you want, you just use the shortcut that launches Quicksilver, (Command + Space in my case) and type in the first couple letters of the thing i want to find, then press enter and it launches. I was as confused as you are about it until i watched a podcast.
If you go into the iTMS and search for the video podcast MacBreak, you'll find a few episodes about it. They are very helpful.
 
I didn't use it for a long time, but I started to recently, and it's really become something I can't work without. Of course, I don't have Tiger right now, so I don't know if I'd use it if I did.
 
Very slick app

It has a LOT of functionality besides application launching, but that's what I mainly use it for. A lot of people have written scripts for it. You can also launch different files/folders, play songs on iTunes, use it as a calculator, etc...All easily accessible with CTRL-space.

You can use Spotlight too to launch apps, but Quicksilver responds far quicker than Spotlight for me. Plus it looks really nice.
 
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