QuickSilver. So far, I'm thrilled. Like my business partner, QuickSilver has freed me from the tyranny of the Dock. Five keystrokes gets me anywhere on my system. I'm particularly enamored with the Terminal plugin : instantly open a Terminal window in any directory on my drive. That plus the Address Book/ Mail integration and I can send emails without ever seeing Mail.app. I'm hooked.
I suggest checking out the Flashlight interface, too. Less obtrusive.