The Shocker

 is a unique variant of the Power Fist with an electric element added to increase its effectiveness against robotic enemies. The Shocker is an an Unarmed Weapon and its damage is affected by the Iron Fist Perk.

Location
The Shocker is found, complete with holotaped instructions, in the Flooded Metro near Arlington Library.

YouTube video for location:
 * http://www.youtube.com/watch?v=sG3r1Z7TbbE