43 Folders has an interesting post asking What is Your Killer App? This is a continuing discussion about how some people get huge productivity from mastering a couple of applications. The comments are filled with what people use. It is heavy on the programmer side of things, but it is still worth a look through.
I would say my killer apps are Mail, NetNewsWire, MindManager, and Textpander. With as much blogging as I do, I would probably add ecto.