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, it lets him or her see things that are not normally visible to the naked eye. It is useful for seeing things such as hidden ladders, invisible platforms and ghosts. It drains Magic Power over time when used.
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.