Quicksilver
It‘s a funny thing; I’ve talked to a few people recently who have said they tried out Quicksilver and didn‘t get on with it. Personally, I very slightly freaked out whenever I’m using a Mac without it. On the other hand, I try to use the mouse as little as possible, so QS is a natural progression (or enabler).
I think part of the reason that some people don't get on with it, is that they only use a few applications, and those are in their dock anyway, therefore QS as an application launcher is of limited use.
Of course, those of us who have been using it a while, know that it can do so much more and therefore directly quoting from "Quicksilver Tutorials Round-up at The Apple Blog, here is a list of tutorials to help you get beyond the whole Quicksilver is an application launcher thing.
Dan Dickinson:
Merlin Mann:
The Apple Blog: