I cannot take this list seriously as it does not include Quicksilver. I put it at number one. It is quite brilliant and has completely changed how I use my mac. I ignore the Dock and have no desktop icons. I can open any file, program, address, mail recipient or just about anything else with half a dozen keystrokes.
If you have a Mac install it now and soend a few hours playing with it and learning. If you have Ubuntu 10.4 there is a very passable clone.