Authors CurseForge

http://wow.curse.com/downloads/wow-addons/details/targetpercent.aspx

Adds a health percentage to the Blizzard target frame.

Wrote it for a few friends, using the addon means you can have raw health enabled on your Unit Frames in the Interface Options, so you can see both percent & raw at the same time.

It's pretty much "done" as-is.

Added support for boss1-4 on request.

Just released a bugfix update, I might let you ALT-drag it next update :)

Quote from Mikari

Does this addon allow you to have the % above the unit frame instead of to the left side, if not would it be possible to have the option?

Are you using alpha? I added alt-drag support if you want to test it out.