If you haven’t been using Quicksilver, you’ve been wasting time. At first it seemed a little ‘too cool’ but now I always expect it to be there ready for me to just start typing. Essentially, your computer waits for a set of keys and if you do those together it pops up a little window and you just start typing. As you type it tries to guess what you need. When it finds it you can just press enter and it will open or run depending on what it is.
And it is super fast.
GNOME Do is fantastic too. I've got it mapped to the same keys Quicksilver used and it's just great.