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.