Ask YC: Essential Mac OSX Productivy Apps

I come from a linux/windows background, but a recent small project requires that I work on OSX 10.5. Being that I am a productivity/efficiency junky, I can't help but feel that I am missing out on something. Can people recommend some essential apps that they use in their day to day workflow? At the top of the list would be a good application launcher which would enable launching a terminal with a keyboard shortcut.