ncMarkTarget is an addon for tanks. It constantly marks the mob you are targetting with a certain raid target icon. This is nice because then the whole raid knows what the tank and the offtank(s) are targetting, and won't attack the wrong mobs.
Install & go! The slash command is either /ncmt or /ncmarktarget.
/ncmt symbol <icon> - Sets the icon to be used for marking. Use one of the following icons: star, circle, diamond, triangle, moon, square, cross or skull. Default is skull.
/ncmt state <state> - Sets when the addon should enable itself. Possible values are: "always", "never" and "instance".