I'm surprised to not see Quicksilver on a list like this. It's definitely the tool that keeps me coming back to OSX. Most of the "replacements" for it on other platforms (and on OSX) implement very few of its power-user features.
I don't really understand the appeal of Quicksilver. If I want to quickly launch some random program, I just press Command + Space (activates spotlight), type in the first two or three letters of the program name, and press enter. You can also do calculations and look up words in the dictionary. Isn't this what most people use Quicksilver and Alfred for?
Most that still use Quicksilver to today are likely not "most people". The "object-action-object" command of QS is not something Spotlight is capable of (e.g. drag files on desktop to trash, open these images in Photoshop, all without involving a mouse click).
I've quit QS for LaunchBar few years ago (when the author announced the development for QS has ceased) then Spotlight then Alfred. I think Alfred did better than Spotlight as a launcher because of slightly better results (e.g. typing "pho" in my machine list Photo Booth and iPhoto but not the one I want the most: Photoshop) and few little touches here and there where Spotlight didn't get it right (e.g. "press enter to copy" for calculations).
I use file actions a lot (move these files, trash this, etc.), so Spotlight is pretty much out of question.
