LaunchBar is one of the tools I can't live without.
Basically, it's an application launcher, but besides actually launching an application it has lots and lots of features that help me complete tasks on my laptop while staying in the flow.
Ever since I installed it on my MacBook my workflow changed quite dramatically to the better.
Most of the time I no longer use ⌘-Tab to switch between the applications.
⌘-Tab requires looking at the list of running applications, and I almost always run more then 10, up to 20 of them, so I need to search for the one that I need and then either Tab to it or choose it with a mouse. A major hit to the flow.
With LaunchBar, if I want to switch to Google Chrome I just do Alt-Space, g, Enter. No need to think, its already in the muscle memory.
Itunes? - Alt-Space, i, Enter. Done.
iTerm? - Alt-Space, t, Enter.
The best part? It learns as you use it. if you type t and its top suggestion is Terminal and not iTerm just use your arrow keys to choose iTerm and after a couple of times it will remember your choice.
This is how I got g, i and t to choose what I need the most.
But this doesn't stop with running applicaitons, starting applications is equally easy. I no longer search in the /Applications folder. If Google Chrome is not running when I hit g it will just start it for me. If I need to run Numbers I just type num, or key for Keynote etc.
Same for directories or mounted volumes. I just type Qdo to go to my Qnup Qdownload volume mount, doc for ~/Document, etc.
Oh, and it can reach inside directories easily. To open our contract with AcmeWidgets Inc. I can do this: drop to choose ~/Dropbox, space to limit the search to its content, ast, space to choose astrails.com subfolder, then acme, space, le, Enter to open the directory that holds all our legal documents with them or space instead of Enter to get a list of the documents and choose a specific one.
In short, I forgot when I had to manually drill down to commonly used directories, LaunchBar is a much better and faster tool for that.
It also integrates with lots of apps i use daily on my laptop, like Itunes, Address Book, iCal and others. So you can play specific track in iTunes, or pause currently playing track, or email someone from your Address Book to name a few.
Also, please, don't even mention Spotlight, its not even close in terms of features, speed and convenience.
Go check it out, you'll thank me later.
UPDATE: After checking out Alfred as was suggested in the comments I actually loved it so much I switched and I'm a happy user for a couple of months now. Recommended!