QuickSilver
This is one of the most useful apps that have been developed for Mac OS X. Its very handy. You can do almost anything with a couple of key strokes without stopping to do what you were doing.
Here is a guide to using QuickSilver.
I also found a talk by the creator of QS. You can watch it here.
I found similar apps for use on Linux. I don’t know how good they are as I have not tried them.
GNOME DO
Katapult
Gnome do. Very good. For Windows, Launchy is good.
I have never used Quicksilver, since I run Linux–but after it was often recommended to me I decided to check out the Linux offerings.
I briefly tried Katapult, but since I run GNOME it didn’t quite fit. When I tried out GNOME Do, though, it was an amazing time-saver. It’s the perfect cross between command line and GUI. I launch just about all the programs I use though Gnome Do, and I use it for opening files quickly rather than opening folders. Very handy.
[Byron] : Very handy indeed. Apart from launching programs, you can write mails, perform quick calculations, browse your filesystem, IM and a lot more without stopping to do whatever you were doing!