Let's say that you're hunting the Voidtalon of the Dark Star. You need to watch out for the very rarely spawning Edge of Reality portals... but it'd help if you had something to yell at you when you brushed by it.
Just type /objectscanner Edge of Reality and every time you mouse over that portal, Illidan will tell you that you weren't really ready for today to be the day.
(But don't bother, because the Edge of Reality is what I made this for, so it's enabled by default.)
Usage
List everything you're currently scanning for: /objectscanner
Toggle an object to scan for: /objectscanner Name of Object
Very important note: capitalization matters. It looks for exactly what you typed to show up as the complete title of the tooltip.
You can track as many objects as you want at once.