This AddOn makes the hit boxes of nameplates you choose to show visually distinct. See the images for previews.
When you choose to show nameplates there is a fixed area that catches mouse clicks for each nameplate.
How does it work?
The AddOn asks to be notified when nameplates are shown. When a nameplate is created for the first time it is assigned a backdrop image. When the nameplate is shown the AddOn sets the alpha channel on the backdrop to 1 to make it visible.
Why doesn't it show for friendly players in dungeons or raids?
This is due to a general restriction added by the World of Warcraft developers.