In no particular order, and I have to cheat:
1. WriteRoom and/or Ulysses and/or TextMate, though WriteRoom doesn't quite like dual monitor setups...
2. Delicious Library
3. Growl
4. CoverFlow and/or MenuTunes
5. Apple's built-in systemwide services (Spell Checking, Dictionary, etc, etc, etc) there's nothing really quite as exquisite.
EDIT: I'm not sure how people can label Firefox as a Macintosh killer app as it's available on almost any platform. If a person were to list Firefox, I'd think Camino would be a better fit as it fits into OS X's mold much, much, much better, being that it's Cocoa.