Having recently joined the 'cult of Apple' - here's what I consider to be the minimum required set of Mac OS X utilities and helper applications for any Mac OS X installation:

1) Quicksilver: Update: 2014-10-11 - One of the features I've relied on in Quicksilver, was the ability to easily set system-wide keyboard shortcust. Thanks to this very cool app - Apptivate which does nothing 'but' set system-wide accelerators (including my prefered keybinding to launch Pathfinder), I'm less reliant on an app launcher now. Spotlight works fine, and Yosemite will finish this off nicely. Original Post: Thanks to This application launcher is the business. It's been around for ages, and does what it says on the tin. It looks a lot like 'Gnome Do', and I love it. A very fast, and very convenient way to launch apps, scripts, open documents and a lot more. It's also free. An alternative to Quicksilver is Alfred - which also looks nice - although I've yet to take it for a spin.