As I've played more with Quicksilver, I've come to the conclusion that if you didn't know about command line terminals and you sat down to design a CLI for a GUI-based machine, Quicksilver's what you'd come up with. At first blush, Quicksilver is a launcher, but its much more than that. It has an adaptive search that targets almost any data source you can think of. What's more, it has a nice plug-in architecture that let's users extend it to apps that might not otherwise get integrated. Some small examples: hitting the hot key and typing "ksl w" let's me launch the KSL Weather page in Firefox. Typing the start of a contact's names brings up their contact information and let's me act on it. I can even select songs to play in iTunes from the keyboard.
One of my personal productivity goals is to reduce the number of times I take my hands off the keyboard and Quicksilver is a big part of making that goal happen. Here are a few resources I've found helpful: