Most people I know who use Quicksilver can't imagine computing without it, and I'm sure that a significant number of users would consider it the single most useful applcation on their computer. What does it do? Well, you can read a detailed description here but basically it's an application launcher on super-steroids. If, like me, you're a keyboard-shortcut kind of guy (as opposed to a slug who doesn't feel at home unless it's drooling all over the mouse), then Quicksilver allows you to launch applications with a few keystrokes. If only for this, it does a fantastic job. But you can also use it to, for instance, select a document and attach it to a mail message addressed to a specific person. No more browsing through your file hierarchy to find the file, dragging it to a new message (that you previously created) and typing in the address. Quicksliver offers an infinitely more elegant and faster solution. It does many other things, and I'm discovering new ways to use it all the time.
Did I mention that it's free? How can something this useful be free? It defies logic. And yet there it is. The developers at Blacktree have apparently decided altruistically that their mission is to make the (computing) world a better place. I'm not sure how they're sustaining the project, but I, for one, am truly grateful. Until a practical voice-based interface comes along (don't get me started about OS X's