Quicksilver
I have to jump on the bandwagon - Quicksilver appears to be brilliant for keyboard users (ie those of us who are less coordinated and find the keyboard more accurate than the mouse)
My biggest problem with it is its install method: it picks up your bookmarks etc. Unfortunately the stuff I use *really* frequently arent in my bookmarks - instead I will alt-tab to safari and then eg typ s-l-a to get slashdot and hit return.
Never mind - its just means I have to customise the catalog. Oh, dear. Using that mouse… *sigh*