Virgin Install: Quicksilver
When I first announced the Virgin Install series the comments were quickly filled with programs that people consider invaluable to their Mac experience. The program most often suggested just happens to be the program that might strike someone new to the Mac as inscrutable.
I'm speaking, of course, of Quicksilver the application that is everything to everyone and one of TUAW's favorites. It is an application launcher, a finder replacement, a script launcher, and much more. A word of caution, though, it has a slight learning curve. When I first installed it I had no idea what to do with it, and I thought the hype was unearned. I gave it some time and now I can't think of using a Mac without it installed.