Lens of Truth
The Lens of Truth is an artifact that lets you see the truth, obviously, and is found in Ocarina of Time and it's sequel, Majora's Mask. The Lens of Truth allows the user to see the truth; that is, to see things that are not normally visible to the naked eye and to ignore fake walls, objects, and enemies. It is also useful for seeing things such as hidden ladders, invisible platforms and ghosts. Be warned, as it drains your Magic Power over time (but heres a trick, set the lens to a button, then tap the button repeatedly and it will not drain your magic).
The Lens of Truth is also refered to as the Eye of Truth and looks very similar to the Sheikah's eye symbol, just without the tear.
In Ocarina of Time, the Lens of Truth can be found in the Bottom of the Well. In Majora's Mask, it can be found in a cave in the Goron Village near Snowhead.
In Twilight Princess Wolf Link has powers similar to those of the Lens of Truth.