For Mac users: Lifehacker has a very helpful post today on ten useful Quicksilver plug-ins. I've been using Quicksilver for most of 2007, and six of these plug-ins were new to me. (I wish I'd known about Shelf months ago: it's already made my Mac life better.)
For non-Mac users: Quicksilver is a free program that launches programs, opens files and folders, and simplifies countless elements of working on a computer. To my mind, Quicksilver alone might be reason enough to switch to a Mac.
Top 10 Quicksilver Plug-ins (Lifehacker)
What is Quicksilver? (Blacktree, Inc.)