It's been around for several years now, and if you use the computer anywhere near as much as we think you do and you don't have Quicksilver, it's time you caught up. Quicksilver is the best productivity app out there, and after a short period of learning and adaptation, takes almost no thought to use.
So...what exactly is Quicksilver? In a nutshell, it turns your keyboard (rather than your mouse) into the most operative element of your computer. With Quicksilver, you can do almost anything from the keyboard: launch applications, upload files to different ftp servers, resize a batch of images and even append text to existing text files (todo.txt, perhaps?). Think about it: you'll never have to navigate to the Applications folder again! Just hit Apple+Space, type the first few letters of the desired app's name, hit Enter, and you're done.
To download Quicksilver or just learn more about it, visit Blacktree, who also host their extensive list of Quicksilver tutorials. But don't be intimidated; even 10 minutes spent learning Quicksilver will make your life infinitely easier. We promise.