Quicksilver reportedly makes people more productive on their Mac OS X computers.
http://theappleblog.com/2005/02/18/quicksilver-changes-everything
http://en.wikipedia.org/wiki/Quicksilver_(software)
For Windows:
http://www.humanized.com/products/
For linux:
http://developer.imendio.com/projects/gnome-launch-box (early stages of
development)