Launchbar or Quicksilver
I haven't tried Quicksilver, but I sure love my Launchbar. As just one example of how I use it, if I want to go online to Wells Fargo Bank, I type Apple-Spacebar and then "well" and Launchbar opens Safari and takes me to the Wells Fargo log-on screen (regardless of what program I am using at the time). But what is really great is that I didn't have to teach it that shortcut. Launchbar learned it on its own.
I gather that Quicksilver is very similar. Quicksilver is free and Launchbar is $20 or $32 for 5 computers in a family.
Why pay money for Launchbar when Quicksilver is free? Well the way I figure it, if someone doesn't help to keep something there, after a while it won't be there. So I am helping to keep Launchbar there. Of course, you could probably donate to Quicksilver, and if you are a fan, I suggest you do that.
Launchbar and Quicksilver compared:
TidBITS Talk on Launchbar vs Quicksilver