I just installed Quicksilver on my Mac and, so far, I am really enjoying it. I have to admit that I don't think I fully grasp what the program can do yet, but here is a description of my limited usage. Basically, Quicksilver is an app for managing how you open programs. For example, once I invoke QS through a preset key combination (in my case, control+space), I can start to type the name of an application I want to run (i.e. calc... to get to calculator) and QS will auto-complete what I am typing. Pressing enter will execute the selected app. If that app is already running, it will switch to it. It's very quick (I guess it's aptly named) and convenient.
This is a great app for keyboard-centric users. It's a nice complement to Nostalgy (see previous post), as well. I see that it supports plug-ins. I haven't had the chance to try any out yet, but I would like one that works with Firefox and Thunderbird to add more keyboard commands. Maybe these already exist.
If you're a former *NIX user or just someone who likes to user the keyboard as your primary input device, I would check this out. Of you're a point-and-click person, probably not worth it.