Basically it's like an App launcher and Spotlight combined, but even more powerful since it uses plug-ins and triggers.
I use it to launch programs quickly (command + space - type first 3 letters of the name of the app + enter).
I have triggers like ^+command + arrow right to skip songs in iTunes.
I use it to find documents and launch them quickly.
I've ditched the dock and have gone totally keyboard-driven (on my powerbook).
You can download it here, the documentation is scarce, but there are some tutorials linked on the site.
http://quicksilver.blacktree.com/
It's not easy in the beginnning, but it's so powerful
