_NPCScan tracks seldom-seen rare mobs by proximity alone. It has the same range as targeting macros, but you won't have to mash any buttons. Just go about your business and let _NPCScan alert you when it finds something.
When a rare mob is found, _NPCScan alerts you by playing a loud and distinctive sound, making your screen pulse red, and displaying an animated targeting button.
_NPCScan's main options window can be accessed from the Interface Options menu. This panel includes a "Test Found Alert" button, various ways to customize the alert sound, and an option to suppress cache warning messages on log-in.