I know I'd be completely lost without LaunchBar on OS X:
I initially thought that entering keyboard commands to run a program was completely opposite what a GUI was supposed to offer, but being a command-line driven guy (hey, I'm getting old!), it was amazingly intuitive, not to mention blazingly fast. I rarely use the toolbar to start programs any more, let alone navigate through the Applications folder.
Definitely recommended for all you OS X folks out there.